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CV16/S79-3/90-rec Serial:  0501
U.S.S. LEXINGTON (CV16)   c/o Fleet Post Office,     San Francisco, California,         28 August, 1945.


From: The Commanding Officer.
To: The Chief of the Bureau of Ordnance.
Subject: Report on Service Experience with Six Caliber .50 Gun Mounts, Mark 31 mod. 0, Forwarding of.
References: (a) BuOrd Speed Ltr. CV16 (Re5d) of 28 June, 1945.
(b) BuOrd Speed Ltr. CV16 (Re5d) of 14 July, 1945.
Enclosure: (A) "Report on Service Experience with Six Caliber .50 Gun Mounts, Mark 31 Mod. 0, aboard the U.S.S. LEXINGTON," dated 28 August, 1945.

    1.         The trial installation of six caliber .50 Gun mounts, Mark 31 Mod. 0 was completed at the Puget Sound Navy Yard on 13 May 1945. Since the date of installation the mounts have been in regular use and 153,000 rounds have been fired at balloons, TDD, and sleeve targets. No anti-aircraft actions involving enemy aircraft have taken place.

    2.         Reference (a) requested a report on the operation of the .50 caliber mounts and recommendations as to how they should be used to replace the present 20mm mounts. This report, Enclosure (A), is forwarded herewith.

    3.         Reference (b) stated that a quad 20mm (N-3) mount had been developed and that installations would commence in the early fall.

    4.         I fully concur with the plan of substituting multiple-gun power-mounts utilizing belt-fed guns (of not less than 20mm in caliber) with high rates of fire for the present free-swinging 20mm mounts. The Mark 31 mount with certain modifications, as listed in Enclosure (A), would be satisfactory. In view of Reference (b) no additional .50 caliber mounts should be distributed, but a high priority should be placed on the production and distribution of the quad. 20mm M-3 mount when found to be satisfactory. Arrangements should be made to install this new mount on ships now operating with the fleet to replace free swinging 20mm mounts.

    5.         This report concerns itself with the caliber .50 gun and mount. It is a technical report. It is not an endorsement favoring the installation of small caliber guns with which to fight a ship. A ship should be provided with an arrangment that can consistently destroy a Kamikaze before he gets within the effective range of either the .50 caliber or 20mms.




Copy To:
S??NAV (Office of Research and Inventions).

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CV16/S79-3/90-rec Serial:  0501
U.S.S. LEXINGTON (CV16)   c/o Fleet Post Office,     San Francisco, California,         28 August, 1945.


Subject: Report on Service Experience with Six Caliber .50 Gun Mounts, Mark 31 mod. 0, Forwarding of.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
DCNO Air (5)
ComServPac (FMO)
ComFirstCarrier T.F.
ComSecCarrier T.F.
ComTaskForce 38
ComTaskGroup 38.1
ComTaskGroup 38.3
ComTaskGroup 38.4
All CV's











28 August, 1945.

























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6 Photographs of the Six Mark 31 Mounts.
1 Photograph of a Sectional Swollen .50 Caliber Barrel.














     1.     Conclusions and recommendations.

           A. One quad .50 caliber mount Mark 31. Mod. 0, has the same overall worth as one twin 20mm mount.

           B. The advantages of the quad .50 caliber mount, Mark. 31, as compared with the twin 20mm mount are:
(1) The higher rate of fire and greater number of barrels result in increased chancesof hitting and in a theoretical greater weight of projectiles being fired at the same target per unit time.
(2) The time required to slew on a fast target and establish a smooth tracking rate is considerably less with the power mount. Inadvertent body and leg movements which hinder smooth tracking with the free-swinging mount do not influence the rate of the power mount.
(3) The unbalanced torque resulting from a gun casualty is overcome by power drive unit of the Mark 31 type of mount and does not destroy the line of aim.
(4) A power casualty on the ship does not put the Mark 31 mount out of commission. With the 20mm unit, a power casualty results in the 1oss of use of the Mark 14 sight.
(5) The belt feed arrangement of the .50 caliber gun provides a longer period of uninterrupted fire, and reduces the likelihood of having to reload during an attack.
(6) A11 of the operators prefer the Mark 31 mount. Their preference appears to be influenced by the fast slewiing and smooth tracking qualities of the mount.
(7) As far as could be determined from the observation of the tracers during firing practices, the .50 caliber fire has been at 1east as accurate and possibly more accurate than the 20mm fire. This accuracy is probably largely due to the excellent tracking qualities of the mount. Greater accuracy would result with a computing sight designed for short range and equipped with a satisfactory device for accurately setting range.

C. The disadvantage of the .50 caliber mount, Mark 31 Mod. 0, as compared with the twin 20mm mount arc:
(1) The .50 caliber projectile does not have the destructive explosive power of the 20mm projectile.
(2) The 1ife of the Stellite barrel, normally used with this mount, is estimated to be 5000 rounds. Under severe firing conditions, however, those barrels swell and burst after three chests or about 600 rounds have been fired. Standard barre1s are unsatisfactory as they wear out causing excessive dispersion after about 1000 rounds have been fired. (These figures are applicable to shipboard AA practices where bursts from 100 to 200 round are often fired.)
(3) The re-loading time, which is about 20 seconds, is several times greater than the loading time for the twin 20mm mount.

D. The Mark 31 mounts require a number of modifications before they would be satistactory for general service use. Most of the difficulties which occured during the first few weeks of operations were due to faulty workmanship or failures of certain parts to withstand the vibration of firing. These faults were correctable aboard ship, but should be corrected in the design of the mount. No failures have occured as a result of corrosion. The batteriesa and gasoline driven generators have perfomed satisfactorily as a source of power.


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E. The utilization of the Cyclic Rate Control Mark 3 was found to be impractical because of the repeated stoppages which resulted from the extractor lug striking or riding over the extractor pivot switch. Better performance and more overall firepower would result from using the .50 caliber gun at the design rate of fire until improved guns with higher rates of fire are available in quantity. Several thousands rounds per gun may accumulate during training excercises between strikes when it is impractical to change all guns or to carry out elaborate preventive maintenance proceduers. The gun performance has been satisfactory since the cyclic rate controls were removed.
F. The following general recommendations are made with regards to subsequent developments, trials, and service installations of power operated light machine gun mounts:
(1) 0nce it has been thoroughly established from proving round tests that the mount is sufficiently effective and reliable to insta11 on a combatant ship and it is desired to determine its effectiveness the ship(s) selected for the trial should be completely equipped with the new mounts. This would not only reduce the difficulty of maintenance and supply for two types of mounts but would make any increase in effective firepower more apparent.
(2) At Least one trained trained gunner's mate, familiar with the mount and the guns should be made available to the trial sbip for each group of four or five mounts installed. This especialiv applies when no shore based training facilities of the type required are available to the ship's personell.
(3) If a power operated quad mount employing the 20mm automatic gun M-3 were used, the increased firepower wouid permit greater spacing between mounts with a resulting decrease in smoke interfernce. Where time and facilities permitted the gallery groups could be completely rearranged to provide a separete circular p1atform, a battle telephone, and a ready service ammunition locker for each unit. Such an arrangement would result in greater angular coverage for mount, better visibility, and less interferance between personell reloading the guns and passing the ammunition. Additional mounts should be located on the island structure, on the fantail and, if possible, on platforms suspended from each cornerr of the flight deck to provide greater firepower in areas most vulnerable to attack.
(4) It is recommended that new types of mounts be equipped with fixed reticle illuminated gunsights with speed rings which may he interpeted in hundreds of knots based on the time of flight of the projectile to 1000 yards, until a computing sight can be developed. The gunsight should be mounted in a shock proof mount to prevent blurring of the reticle image and to increase bulb life. A computing sight should be developed with high priority.
(5 ) It is suggested that an aided tracking power unit be developed for use in future mounts. It should be possible to remove and replace the power unit without removing the entire mount from its foundration.
(6) No further installation of this power mount should be made until 20mm guns replace the .50 caliber guns. It is requested that facilities be provided for conversion of the LEXINGTON mounts as soon as practical.
(7) The quadruple 20mm mount should replace two twin 20mm mounts or four single 20mm mounts.


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     2.     Material.

A. All of the material listed in Enclosure (a) "Assembly list of Ordnance Material" of Bu0rd ltr. CV-16 (Ad7b) dated 10 May 1945 was eventually received with the exception of the mount tools and Electric Trigger Controls. The Electric Trigger Controls were shipped folloving a dispatch request from the LEXINGTON. (instruction manuals, blueprints, a list of material shipments, and much of the actual material for the LEXINGTON installation were received by the ship several days after the scheduled sailing date and after the mounts had been installed and test fired.)

B. Most of the spare parts included in the Mark 31 Mod. 0 allowance list, NAVORD LIST NO. 21522, dated April 9, 1945, were not and are not available to the ship. When this material is made available the ships concerned should be notified.

NAVORD LIST NO. 21522 is made out on a "per mount" basis which is satisfactory where only small numbers of mounts are installed on each ship. If a large number of mounts were used it would be high on spare assemblies such as the drive and power charger units and low on wearable and breakable parts such as piston rings, valves, pulleys, etc. It is suggested that a column B list based on a group of mounts be made up to include complete spare assemblies and that the column A list be revised accordingly. It is further recommended that one complete mount be supplied to each ship for training and emergency replacement. (One mount was destroyed on the LEXINTON as a result of an airplane crash during routine landing operations. There was no replacement .50 caliber mount available.) The fo11ing comments are made in conection a11owance lists, NAVORD LISTs NOS. 21521 and 21522:
(1) Two spare batteries per mount should be included. The batteries are of a standard type (Exide 4H or Delco 19C2) but are not available in Naval Supply Depots.
(2) The Safety Belt G103-1518302 and the Vertical Yoke Adjustment assemblies G102-6294352,3 should be omititted. (A diffirent type of front mounting yokc is used.)
(3) A tachometer for setting the R.P.M. of the power charger after overhaul should be included for every group of mounts.
(4 ) A complete set of power charger spares should be made available for each group of mounts. Chargers wi11 require a partial overhaul including primarily carbon cleaning and valve grinding after about every 100 hours of opertion.
(5) One Set of NAVORD LIST NO. 20968 Column B spares should be included with each shipment of spare parts to ships receiving mounts for the first time. This list shou1d include one spare alternate feed bolt assembly for each two guns.
(6) The manual TM9-1223 should be included with TM9-223. It is strongly recommended that a new operator's manual covering maintenance and opertion of the mount and the guns, and use of the fixed reticle sight be prepared the applied Panel of OSRD, if the mounts are to have wide distribution.
(7) A special tool kit which would include all of the necessary wrenches, screwdrivers, hydrometers, headspace and timing gages, and machine gun tools should be provided for each mount.
(8) It is believed the a1lowance of sixteen (16) special barrels per mount is satisfactory if spare barrels are to be made available in forward areas.


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D. Because of the many improvements and modifications of the origina1 .50 caliber machine gun it would be highly desirable if only guns of recent manufacture were made available for these mounts. It is also recommended that only guns in moistture proof wrappings be provided. Degreasing a large number of guns is very difficult to do on board ship especially if guns have to be made ready for use a short time after they are drawn.
E. A complete set of instructions and an assearibly list of should be made available to the ship and the yard where the mounts are to be installed as early as possible and preferably two weeks in advance.
F. The following miscellaneous items were obtained from Naval Supply Depots or other sources and were found usefull in maintaining and opererating the mounts:

No.                   Item Stock (or other) No.
6 Sight, Mark 9 Mod. 1 (spare) 2-S-3115-25
10 Gunsight (Diffraction ring) 2-S-3380
150 Lamps, 12 volts, 6-21cp 17-L-2145
1 Set Parts (special) for 6
B. and S. Power Charges
6 Batteries, 6 volt, 150 a.h.     -
6 Can, gasoline, 5 gal. 7D-500
6 Cans, Oil Filler, 2 Qt. 13-F-920
6 Cans, Oil Pump, 1 Qt. 13-0-920
12 Bolt Assemblies (.50 Cal.) 1-S-2801
2 Modified (Chinn) .50 Cal. gage kits     -
2 Wrench, combination 1-W-1955
8 Reflector, Barrel 1-R-1098
1 Chart, instruction 1-C-3422-80
1 Training film, (Army)-Mount TF4-1280
1    "      "      "    Drill TF4-1292
1    "      "      "    0.50 Cal. MA503c
1    "      "      "      "   " MA503e
1    "      "      "      "   " MA503a

     3.     Installation.

A. The installation of the six quad .50 caliber mounts by the Puget Sound Navy Yard was completed on 13 May 1945. This included the installation of the adapters, mounts and the cutting of the firing interruptor cams. The insta11ation and boresighting of the machine guns was carried out by ship's personnel. The 10" mount stand was left in place between the mount and the adapter for the fol1owing reasons:
(1) Since the arrival of all neccessary material was uncertain it was belived advisable to leave the Mark 14 sight power connector in place. The connector interferes with the powerdrive unit when the unit is lowered into the adaptor.
(2) The height of the adaptor face was only 13" above the deck, making the height of the e1evation axis only 391/2" above the deck. This height is sufficient to permit the guns to fire over the 24" shield when trained on the beam and to compensate for an angle of roll of up to 11º regardless of the train ang1e when firing horizonta11y. Such an arrangement (using the 13" adaptor without thie stand) would not prevent the guns from firing into the shield when trained, fully depressed, away from the beam. A "safety circle" with a 48" radius was chosen for the adjoining


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2Omm twin when cutting the cams. This permits a train angle of 59º away from the beam toward the adjoining mount. The elevation axis must be at least 481/2" above the deck if the shield is to be cleared at this maximum firing angle*. It is 491/2" with the stand and 391/2" with the stand removed.

* Note: The perpendicular distance "b" from the mount to the shield is 60". The height of the elevation axis above the lower gun axis is approximately 31/2". If "D" is the maximurn depression angle relative to the deck, and "A" is the train angle measured from the beam, then "h" the height of the lower guns above the top of the shield is given by:

h≅b tan D sec A

B. The mounts were all located on the gallery walkways as shown in the attached photographs. (The stands are removed in the photographs.) The locations of the mounts are given below:

Mount Ser. No. Group No. Position in Group       Frame
  1  61788    1    Forward 2 3/4 Stbd.
  2  61793    2    Forward 2 1/2 Port
  3  61789    3    Aft 32 3/4 Stbd.
  4  61787    4    Aft 62-3/4 Port
  5  61805    9    Aft 155-1/4 Stbd.
  6  61790    8    Aft 159 Port

C. The average unrestricted firing angle in train of each mount is 110º, about 60º measured from the beam toward the adjacent 20mm mount in the sponson and 50º from the bean in the opposite direction. The latter angle is 1imited by the raised section in the splinter shield.
D. It was found extremely difficult for some loaders to load the chests because of the added height through which the chests must be raised when both the stand and the adapter are used. The stands were removed, with a consequent loss in depression angle, at Pearl Harbor on 4 June 1945. This necessitated the removal of the sight power connectors. Bcfore the P.S.N.Y. adapters could be used they had to be removed and taken to the Pearl Harbor Navy Yard where they were faced on the top and bottom surfaces in a vertical lathe. This was necessary to prevent severe warping of the azimuth ring gear. (The adapters were apparently welded or stress relieved after machining.) The access door inner projections were cut away with an oxy-acetylene torch to prevent interference with the gear boxes of the drive mechanism. The eccentricity of the bolt circles on the adapters and the oversize outside diamters prevented the further use of the N.G.F. splash shields around the base of the mount. It was neccessary to use a longer elevation stop adjustment bolt and reduce the gun depression angle to about 4º after the stands were removed.
E. The present ready-service ammunition arrangement provides for twenty-two magazines of 20mm ammuniton per barrel and six chests of .50 Caliber ammunition per barrel. Racks for holding 24 chests of .50 caliber ammunition eere installed by the Puget Sound Navy Yard. These racks measure roughly 40" wide and 30" deep, and occupy about 50 cu. ft. of volume in the ready service room. The details of the .50 ca1iber racks should be requested from the Commandant, Puget Sound Navy Yard.


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F. Mounts were damaged on two occasions as, a result of air operations.
(1) on 6 June 1945 Mount No. 5, Ser. 61805, was covered with flaming gasoline, foamite, and salt water as a result of an airplane crash on the flight deck. The mount cover was burned and throvn overboard. The gunsight and firing solenoid wire insulation was slightly damaged but the mount remained operative. No serrious damage resulted.
(2), On 8 July 1945 Mount No. 6, Ser. 61790 was damaged beyond repair when an aircraft crashed on the gun sponson during landing operations. The mount was removed and stripped of usable parts for spares. A twin 20mm mount was installed in its place. Useful note for modelers! See the photo of Mount six below if you need help figuring out where this was.

     4.     Recommendations on Group Arrangement.

Although it is not anticipated that additional quad .50 caliber mounts, Mk. 31, will be distributed, the following recommendations are made with regard to group arrangment as they apply to some extent to any multiple-gun power-mount:
(1) It is not possible to achieve ease of loading and full depression of the guns at all angles with the same adapter. There appear to be only two ways in which all conditions can be satisfied. First is to use an adapter with the upper face about 24" above the deck, and use a 12" platform on each side of the mount for the 1oaders. The second and more desirable method would be to rearrange the group using separate semi-circular platforms of 72"* radius outboard of the longitudinal centerline of the mounts. The spacing of the mounts would probably depend on the permissable topside weight but might depend on the total lenpth of the group or groups. The maximum number of mounts might be limited by spacing in the groups. This spacing should never be less than 150"**. Such an arrangement would have many advantages with ease of loading, decreased interference in passing ammunition, a greater degree of safety, improved visibility, etc.

* Note: Sixty inches would be a minimum radius.

** Note:  At this distance the product of unrestricted firing angle and the number of mounts in a group of fixed length is a maximum. This assumes the radius of the safety circle of the adjoining mount to be 72". Thc total train firing angle of a mount in the center of the group is: 2A = 2 cos - 1 R-r/d, where "R" is the radius of the safety circle "r" is gun distance from the center, and "d" is the spacing between the centers of the mounts.

(2) A practical arrangement would be to replace two twin mounts with one quad 20mm power mount.
(3) If .50 caliber mounts are to be used on other trial installations, the ready service ammunition lockers should be rearranged to provide for stowing ammunition chests and at least 100% spare machine guns. On Group No. 1 on this vessel, for example, there are two rooms which measurc 63 1/2" in depth, 77 1/2" in usable height, and 202" and 191" in width. If Group No. 1 were rearranged to accommodate four (4) Mark 31 mounts these two rooms could be rearranged to hold 48 chests of ammunition and 60 boxes of linked ammunition totaling 25,500 rounds per room or about 3,200 rounds per barrel. This would give about 4 minutes of firing time per barrel which is the same firing time permissable with the present 20mm arrangement.


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This arrangement would also permit the installation of two 60" X 30" work benches in each room, i.e. one for each mount operator.

     5.     Mount Design.

A. Mount casualties. The following is a summary of the mechanical and electrical failures which have occurred on the mounts sinco they were installed. Many of the items were of an easily corractable nature. Only a small number of the failures caused the mounts to become inoperative during firing practices.

Sheared elevation taper pins. Fatigue   5
Broken torque tube shaft. Fatigue - no fillet   1
Frozen firing relay. Design   4
Gunsight lamp failures. Vibration               Approx.  30
Loose setscew on 1:1 gearbox
pulley Gear hub
Workmanship   1
Frozen triggor switch. Design   3
Loose ammo bolt guides. Vibration - design   6
Excessive drift - loose pulley
set screws.
workmanship   1
Excessive Drift - poor power
unit alignment.
Workmanship   2
Loose sight lamp ass'y. Vibration - design   4
Gunsight "day" contact. Vibration - design   2
Slow power charger - loose
adjusting nut.
Workmanship   1
Slow power charger - valve
leakage and carbon.
Routine usage  10
Bent azimuth pinion shaft. Personnel   1
Damaged battery cases. Personnel   2

B. Changes in design aboard ship.

The following is a summary of the changes which were found necessary and which were made aboard this vessel:
(1) The Spray Shield BuOrd Dwg. 506971, was removed because of interference with the adapter.
(2) Diffraction ring standby sights were installed on and boresighted with the Mark 9 Mod. 1 sights. They were found necessary because of the short lamp life in the Mark 9 Mod. 1 Sight.
(3) The Cyclic Rate Controls, Mark 3 Mod. 0, vere removed after a total of about l0O000 rounds had been fired from the six mounts, because of the high frequency of stoppages as a result of interference between the extractor lug and the extractor pivot switch.
(4) The Shield Securing Locks, BuOrd Dwg. 506965, were removed because they failed to support the weight of the armor shield and became loose duing normal usage. The armor shields were bolted in place.
(5) The ammunition belt guides which all loosened from the vibration of firing were aligned and bolted in place with 5/16" bolts and elastic stop nuts.
(6) Oversize taper pins (No. 1) were used for replacing the existing taper pins on the elevation pinion gears. The existing pins on five of the six mounts failed in shear.


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(7) Voltmeters were removed as they did not serve to give a reliable indication of the remaining energy in the batteries and would not withstand the vibration of firing.

C. Essential modifications. The following modifications are essential to the succesful operation of the mounts:
(1) An improved easily adjustable and easily removable firing circuit relay should be used. This is particularly important since the firing switch is in the primary circuit. The relays (A 343599) caused trouble on several occasions by remaining closed when no longer energized.
(2) The sharp reduction in cross section at each end of the torsion tube (shaft C 130530) should be filleted to increase its resistance to fatigue. One shaft failed as a result of a fatigue crack which began at the face where the cross section was reduced to facilitate mounting the 1" - 12 tooth pinion. It is further recomended that the shaft be made of an alloy steel with high strength and toughness.
(3) Taper pins of a slightly larger diameter should be used for securing the elevation pinion firmly staked in place. It was necessary to replace pins that had failed in shear on five of the six mounts.
(4) A front gun mounting yoke of a simplified and more substantial design is recommended. The spring loaded locking pieces came loose on several of the yokes as a result of vibration. These locking pieces are held in place by a press fit pin which does not extend through the yoke. The eccentric locking feature is neither workable nor desirable..
(5) ammunition belt guides should be both aligned and bolted securely in place at the factory. It was necessary to bend a11 of the guides through a large ang1e to a1ign them with the feedeyes of the guns. They loosened repeatedly from vibration and eventually were bolted in place with 5/16" bolts and elastic stop nuts. The guides caused no more trouble after this was done.
(6) The armor shield locking arrangement (BuOrd Dwg, 605965) should be redesigned for strength and non-interference with the trunnion sector gear. A11 armor securing locks bent from normal usage. Armor shields have been bolted in place with 3/8" bolts.
(7) An improved waterproof type of gunsight to be mounted on a shockproof adaptor is reccomended. This sight should be of the illuminated type and should have rings for 100, 200, 300, 400 and possibly 500 knots with the knot-mil ratio based on the time of flight of the projectile to 1000 yards. The fol1owing modifications of the Mark 9 Mod. 1 sight would be necessary before it could be considered satisfactory:
(a) A shock proof mounting for the entire sight.
(b) A detent for the wiper arm to hold it in the "day" position.
(c) A steel angle in p1ace of the aluminun alloy angle to engage with the release button for the dimmer-switch assembly to prevent the assembly from becoming loose or falling out during firing. The assemblies were tied in place with cord or wire on several sights before they were replaced. An Amphenal Connecter should be provided on the mount to facilitate easy replacement of the gunsight.
(8) Voltmeters should not be used on the mounts. They fall apart after a few days of firing. They also give a false impression of the energy in the battery. We understand that voltmeters are not included on mounts made after 16 March, 1944).

D. Modificaticns applicable to future models. Future models of this mount should be redesigned with an emphasis placed on increasing the safety factcrs on all parts where necessary, especially in the drive system.


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the following list contains recommended changes applicable to future Marks or models.
(1) A mechanical contour cam device for prohibiting the gunner from training the mount into restrictect areas should be provided. This would greatly increase the coverage of the mount, especially when it is mounted in a gallery, group. It would afford more protection to personnel on adjoining guns when "cook-offs" are likely.
(2) The leather dust shield for the azimuth ring gear should be held in place with metal strips provided with metal "fingers" to insure a good seal.
(3) A dead-man switch would reduce the operating time of the mount and the charger. "Idling" of the power drive unit would then require a conscious effort on the part of the operator.
(4) The present V-belt pulleys are easily bent or damaged. They should be of solid construction and securely fastened to the pulley shafts. A slight inclination of a pulley results in a tendency for the mount to drift.
(5) The V-belt drive differential adjusting slots should be lengthened to permit further tightening of the V-belts. If a separate cover for the slots held in place by scews were used it would be unnecessary to remove the large cover plates (BuOrd 506967) which make's V-belt adjustment a major item in maintenance.
(6) A spring loaded zero-centering adjusternent device mountcd on the gear support lever would facilitate a corrective measure for drift. Elevation drift to some degree was present in all mounts and could not be completly removed.
(7) A square spline shaft to replace the present involute spline would strengthen the power drive system.
(8) A two cycle power charger of 60-90 ampere capacity is reccomended. If a charger of this capacity were used, switching from the "low" to the "high" connections on the generator field windings should be accomplished by a relay operating from the drive motor circuit. Such an arrangement wou1d insure a maximum charging rate when the mount was operating but would prevent overcharging and overheating of the cells. (If ship's power were used as the main source of energy and a motor-generator or dry rectifier were used it would be advantageous to retain the batteries as an emergency source of power).
(9) An electric charging mechanism for the guns or an electrically operated device for holding the bolt to the rear would facilitate more rapid cooling and e1iminate the ever present danger of a personnel casualty from a "cook-off" or from the explosion of a hot round which has been removed from the gun.
(l0) Although the back plain solenoids have been satisfactory it is believed that a top plate or side plate solenoid would be preferable for obtaining more accurrate timing.
(ll) A thicker and stronger gasket should be used on the inner trunnion cover. A better seal would be provided if a stiffener ring were used between the screws.
(12) The indicator lamps and motor thermostat are believed superfluous and unnecessary. Their removal would simplify the wiring arrangement.
(13) A special wrench should be provided for turning the boresight handwheels which cannot, in most cases, be turned by hand. A redesign of the mountings to permit the use of an ordinary large end wrench would be desirable.
(14) Manual triggers on the guns are easily broken and are believed to be unnecessary.


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(15) A sheet metal heat insulator should be provided between the gunner's back and the power charger unit.
(16) The armor shield should be enlarged to provide more footroom when entering and vacating the mount. More knee room should also be provided. Constant rubbing of the knees against the armor shield becomes painful when the operators must remain in the mounts for long periods of time.
(17) The mounting holes on the azirnuth ring gear should be radially slotted to permit the insertion of the mounting bolts without disconnecting the azimuth pinion shaft ang rotating the mount. A slightly 1arger bolt circle would accomplish the same purpose.
(18) A redesign of the sight and mount cover is recommended. A loose light weight cylindrically shaped mount cover with a strong tie rope threaded through a hem at the bottom is recommended. The N.G.F. power charger cover and frame modification (BuOrd Dwgs. 506966 and 506969) were found to be very satisfactory. It is believed that if a quickly removable and replace-able mount cover were supplied the other protective covers would be unnecessary.
(19) A permanent metal splash cover should be provided for the exposed generator windings.
(20) An easily removable electric aided-tracking power unit should be developed to replace the V-belt unit.

     6.     Gun Performance.

A. Rounds fired. Between 14 May 1945 and 24 August 1945 153,000 rounds of 0.50 caliber ammunition were fired on thirty-nine firing days. Not a11 mounts fired on every firing day.

B. Quality of guns. Most of the guns received in the initial shipment were guns of old manufacture. There was evidence of wear in some of the guns which had been presumably reworked. The guns were changed on 25 June 1945 when almost 100,000 rounds naa been fired from the 24 guns. Relatively new guns were used to replace the old guns.

C. Removal of Cyclic Rate Controls. No Cyclic Rate Controls were installed on the new guns on 25 June 1945. The chief reason for the non-use of the rate controls on the new guns was the increasing number of stoppages which had occurrcd as a result of the extractor lug burring or riding over the pivot switch on counter-recoil on the first installation of the guns.

D. Casualties. The following is a partial list of gun casualties which had occurred up to 21 July 1945. Nearly all of the casualties with the exception of the barrel explosions occurred on the initial installation of guns, prior to 25 June 1945. The list is by no means complete. It will however reflect to some extent the type and relative frequency of certain casualties.

Extractor lug striking corner or upper slope of pivot switch.     7
Broken ejector pin. 5
Barrel cxploding forward of liner. 4
Incorrect seating of liner (binding projectile). 2
Broken Accelerator. 2
Worn firing pin. 2
Enlarged firing pin hole. 2


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Warped retracting slide ass'y. 2
Oversize "T" slot. 2
Broken back plate bufffer. 1
Loose breech lock depressor pin. 1
Broken rear cartridge stop. 1
Broken sear. 1
Broken barrel jacket. 1
Broken driving spring rod. 1
Broken firing pin. 1

E. Barrel life. No accurate data has been obtained with regard to the barrel life of the special barrels under service conditions. The barrels have received severe treatment because of the frequent anti-aircraft practices. Gunners have not only been encouraged but have been instructed to shoot continuously when the target is in range. Two hundred round bursts have been fired frequently and often in succession. The following table of four exploded and two "swollen" barrels* indicates that the barrcls will not withstand three chests or 600 rounds in succession, even when new, unless some anpreciable cooling period is allowed between bursts.

   1085       850
   3800       275
   1860      1175
   4400       950
    800       600
    800       600

* Note: A cross section photograph image of one of the swollen barrels is attached herewith.

The average barrel life is estimted at 5,000 rounds, whereas the life of an individual barrel might vary frcm 600 rounds to well over 5,000 rounds depending on the number of rounds fired with the barrel above the critical temperature and the maximum firing temperatures reached. The guns were equipped with standard barrels prior to 1 June 1945. These barrels were used to conserve the special barrels during the initial training period. The projectiles began to tumble after 1000 rounds had been fired from the standard barrels.

F. Personnel Casualties. The most common personnel errors were:
(1) Pulling too hard on the retracting slide handles and breaking same. at least ten of the old type of handles were broken.
(2) Replacing the back plate on the run with the firing solenoid "flipper" up. The stoppages resulting from incorrect headspace, or incorrect oil buffer adjustment, were relatively negligible.

     7.     Personnel and Training.

An officer on temporary duty from the Special Devices Division of the Office of Research and Invention, Lt. E.V. Hardway, USNR, was (and is) of greatest assistance in the installation details, maintenance, and the training of the gunners and loaders. He was ably assisted during the first six weeks by Army Private (formerly a civilian technician), Gordon Anderson, Pfc. U.S. Army Air Corps.


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No ship's company officers or men had previous experience with this mount and installation and training would have been very difficult without the services of these specialists.

B. The four forward mounts were manned by strikers and loaders of the 5th (forward l.m.g.) division with a gunner's mate second class in charge of maintenance. The after mounts were manned by privates of the 6th (after l.m.g.) division. The wide dispersion of the mounts about the ship among six different groups made training and evaluation of results several times more difficult than it would have been if the mounts had been grouped together. Insufficient mounts were installed to warrant forming a new division.

C. The initial training consisted of lectures and the showing of training films and slide films during both day and evening periods. This trainig carried on at the end of a Navy Yard overhaul period was under very trying conditions, due to loading ship, inspecting installation of material, and cleaning ship. During the first six weeks anti-aircraft firing exercises were conducted almost daily. The men thus received considerable training through experience and informal instruction in tracking, firing, gun cleaning, mount maintenance, etc.. This training would have been far more efficient if more time had been available for analysis. Training accomplished on board durirg the first six weeks could have been accomplished in a much less time at a shore based school, with adequate facilities. Every effort should be made to train men on shore, for a new type mount, before the men are ordered for duty with the mount.

D. A marked improvement resulted after a program of routine maintenance was commenced. This consisted of daily inspection "Check-Offs" by the mount operators, weekly and monthly inspections by the gunner's mate and gunnery sergeant in charge and frequent random inspections by the responsible officer. A record of inspection, rounds fired, casualties, mount history, etc. is being kept in a gunnery office log. The mount operators turn in a report of rounds fired through each barrel and gun and mount casualties after each firing exercise.

E. One loader was injured as a result of violating the safety precautions. Six hundred rounds had been fired through each barrel in a re1atively short time. The guns were unloaded immediately after the firing run. Orders were then given to stand by for another firing run. As a result the guns were reloaded and the target was tracked but "commence firing" was not given. The left loader then unloaded the guns and picked the unexploded cartridge up to throw it over the side. It exploded in his hand causing burns on his hand and ankle about two seconds after it was ejected. The loaders had been previously warned to leave the gun loaded in such a case but to clear the chamber immediately after the first "cook-off." The loaders are required to clear the chamber immediately after each firing run. (This was done in the above case but the guns were reloaded a few seconds afterwards.) It is believed that it will be very difficult to entirely prevent such personnel casua1ties on a ship fully equipped with Mark 31 mounts unless some means is provided for holding the bolt to the rear automatically.

F. If a quad 20mm (M-3) mount is to be used it is recommended that a gunner's mate - mount operator with at least 2 to 3 weeks of specialized training ashore to placed in charge of each mount. At least four men, one or two being strikers, should be used for loaders. At least one specially trained gunner's mate for each group of four or five mounts should be made available to any ship chosen for a trial installation.


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G. In the event that sufficient facilites are not available at AATC's for conducting training courses in the quad .50 caliber or 20mm mount, it is suggested that arrangements be made for utilizing the facilities of the Naval Air Gunner's Schools and Combat Aircrewman Training Units.

F:. Inspection "Check Off 11 list.
The following is a list of items believed to be essential for inclusion in a routine inspection report. The "daily" items are checked each day by the operator and once a week by the gunner's mate in charge of the mounts. The "week1y" items are checked weekly by the operator and are included in the gunner's mate's weekly check. The "monthly" items are carried out and inspected once a month. The list of items is essentially the same as those included on a mimeographed form which has been used for routine inspections.

l. Mount operation-elevation and train.
2. Gunsight-day and night filaments.
3. Operation of firing circuit and solenoids.
4. Firing cut-out circuit, elevation and train.
5. Guns - clean and lightly oiled.
6. Barrels - clean and free from "swelling" or severe erosion.
7. Headspace adjustment correct.
8. Backplate adjusting screws tight, 40-60 1b. - ft.
9. Securing wires on solennoids and retracting slides.
10. Tightness and security of handwheels.
l1. Operation of power charger on "low" and "high".
12. Crank case full of oil.
13. Gas tank full.
14. Oil in air filter at correct level.
15. Solution leve1 in battery correct.
16. Specific gravity between 1275 and 1285
17. A11 ammunition chests loaded.
18. A11 exterior surfaces painted or greased and rust free.
l. Complete careful inspection and cleaning of guns. Oil buffers filled.
2. Guns boresighted correctly.
3. Four complete spare guns, two right hand, and two left hand.
4. One complete spare bolt for ready replacement in spare parts box.
5. Universal joints and splines oiled.
6. Power charger crankcase oil changed.
7. Oil In air filter changed.
8. Check tightness and adjustment of V-belts.
9. Check tightness of elevation pinion gears on shafts.
10. Check pinion gears are sector gears for rust - grease if necessary,
l. Check oil level in train and e1evation differentials.
2. Oil trunion bearings.


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Mount 1
Mount 2
Mount 3
Mount 1, Frame
2-3/4, Starboard
Mount 2, Frame
2-1/2, Port
Mount 3, Frame
32-3/4, Starboard
Mount 4 Mount 5 Mount 6
Mount 4, Frame
62-3/4, Port
Mount 5, Frame
155-1/4, Starboard
Mount 6, Frame
159, Port
Swollen .50 Caliber Barrel mentioned in section 1, C. (2)

National Archives & Records Administration, Seattle Branch
Record Group 181, Ship Files ca 1940-1950

Transcribed by RESEARCHER @ LARGE. Formatting & Comments Copyright R@L.

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