"0.50-Inch Machine Gun Mounts, Mark 17 Mods. 1 and 2", 1943, covers the PT boat manual 50 caliber gun mount.
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ORDNANCE PAMPHLET NO. 951
0.50-INCH MACHINE GUN MOUNTS
MARK 17 MODS. 1 AND 2
This publication is RESTRICTED and will be handled in accordance with Article 76 United States Navy Regulations, 1920
BUREAU OF ORDNANCE
WASHINGTON, D. C.
ORDNANCE PAMPHLET NO. 951
0".50 MOUNT, MARK 17 MODS. 1 AND 2
1. Ordnance Pamphlet No. 951 was prepared from Bureau of Ordnance drawings, and the material furnished by the Bell Aircraft Corporation and the Electric Boat Company.
2. The 0".50 Mount, Mark 17 Mod. 1 differs from the Mark 17 in the use of a new track bearing and a new brake, as well as the addition of a horizontal stowing latch and canvas covers for the ammunition boxes.
3. The 0".50 Mount, Mark 17 Mod. 2 is the MO Mount, Mark 17 converted to include the changes incorporated in the Mark 17, Mod. 1. The Mark 17, Mod. 2 is identical to the Mark 17, Mod. 1 except for the fixed bearing race and the attachment of the outer bearing shield. This conversion affects the carriage only, which becomes Carriage Mark 9, Mod. 2, instead of Mark 9, Mod 1.
4. The text includes references to the following publications:
O.P. 716 (War Dept. SNL A36)
5. This publication is restricted and should be handled in accordance with the provisions of Article 76, U. S. Navy Regulations, 1920.
1. The 0".50 Mount, Mark 17 Mod. 1 is a hand-operated
machine-gun mount, carrying two caliber 0.50 Browning aircraft machine guns. This mount is also designated 0".50 Assembly No. 41, and consists of the following major subassemblies:
List of Drawings Sk. No.
General Arrangement Dr. No.
0".50 Carriage, Mark 9 Mod. 1
0".50 Cradle, Mark 9
0".50 Sight, Mark 11
2. The principal features distinguishing this mount from
the earlier 0".50 Mount, Mark 17 are these:
(a) Train bearing of new design.
(b) Hand brake of new design.
(c) Horizontal stowing latch. This is an added
feature and is used to hold the guns secured in horizontal position.
3. Operation characteristics and installation dimensions of the subject mount are identical with those of 0".50 Mount, Mark 17. The heavy brake ring has been eliminated, and the brake shoe of the new brake operates against the outside of the fixed ring.
4. The carriage of the mount provides unlimited train
which is accomplished by the operator exerting pressure, in the direction desired, on a winged back rest attached to the rotating carriage. (See pl. 2.)
5. The gun cradle is supported on trunnions by a yoke
which is pivoted on the support structure. This arrangement
permits the guns to be manually elevated, depressed, and traversed
within the limits set, irrespective of the position of the carriage in train. The maximum elevation is 85°, the maximum depression is 15°, the traverse movement, i.e., the rotation of yoke axis, is limited by stops from 10° to 15° to either side of the center line, depending on elevation of guns. Depression of guns is limited by a circular rail supported by a protective shield or by other suitable means. This depression stop prevents the guns from being pointed at the ship's personnel or structure when the gunner is firing over the boat. (See pl. 1.) The depression-limit stop rail is normally installed by the shipbuilders and shaped according to the mount location and ship's structure.
6. For information on operation and cleaning of gun,
see O.P. No. 716 (War Dep't. SNL A 36).
Weight of the Mount - without guns, ammunition or protective shield
Weight of two guns
Weight of 1000 rounds of ammunition in links
0.50-INCH MOUNT, MARK 17 MOD.1-INSTALLATION ON BOAT
0.50-INCH MOUNT, MARK 17 MOD.1-SECTIONAL VIEW
0.50-INCH MOUNT, MARK 17 MOD. 1-FRONT VIEW
0.50-INCH MOUNT, MARK 17 MOD. 1-PLAN VIEW
0".50 Carriage, Mark 9 Mod. 1
1. The carriage of the mount consists of a fixed ring
with a flange which is bolted down to the boat's structure, and a rotating structure which supports and carries the cradle, the ammunition-box supports, the spent cartridge and link chutes, a back rest, a counterweight, a hand brake, and the stowing and securing latches. The rotating structure of the carriage is supported on a large rotating ring and the fixed ring; the two rings compose the roller-bearing path for training the mount.
2. The train bearing consists of a single complement
of cylindrical, solid rollers. Assembled in the raceways are 105 rollers caged in 15 circular separator strips, each strip holding 7 rollers. (See pl. 5.) The axes of the rollers are inclined 45° with respect to the axes of the raceway rings, but in each separator strip the axis of one roller assumes alternate position with respect to the axes of the other six rollers. Consequently, in the train bearing 15 rollers take the upward load and 90 rollers take the downward load of the carriage.
3. The hand-brake assembly is bolted on the pads provided on the rotating support ring (see pls. 4 and 6). With brake handle locked in horizontal position to the left (with reference to operator inside the turret) the brake shoe should be clear and free from the brake surface, i.e., the fixed ring of the carriage.
4. To hold the mount in train in any fixed position, turn the brake handle 180° to the right for maximum braking effect. The hand brake may also be used to apply varying degrees of braking effort by engaging the brake-handle detent in any one of the three intermediate notches provided on the cam surface of the brake bracket.
Ammunition Boxes and Cover
5. Ammunition to the guns is supplied from four boxes, two per gun, each having a capacity of 250 rounds, or 500 rounds per gun. Ammunition boxes are carried on supports attached to the rotating carriage structure. Ammunition should always be
packed in the boxes so that the double loop of the link will be leading and, therefore, will be fed into the gun first. This is important because the feed mechanism of the gun will operate with a greater belt load if the double loop is fed first, for then both the cartridge-feed pawl and the holding pawl will always bear against a single thickness of the link thereby properly spacing the belt.
6. Ammunition boxes should be loaded so that the end of the first row of cartridges laid on the bottom of the ammunition box will extend up the rear end of the box with three or four cartridges hanging over the top of the rear of the box. Thus, the tail end of the belt from the front box can be connected to the front end of the belt in the rear box.
7. From the boxes the ammunition passes through lower and upper ammunition guides to the guns (see pl. 3). When the front box is empty the ammunition will feed from the rear box. Where time permits, however, the empty front box should be removed, the rear box slid forward, a full box replaced at the rear, and its belt connected to the end of the belt in the front box.
CAUTION: As the aircraft gun is used on this turret, initial bursts should be limited to 75 rounds, with a desired rate of about 20 rounds per minute thereafter. Caution should be observed against possible "cook offs" of the round in the barrel chamber of a hot gun.
8. To protect the ammunition from the weather, a canvas cover is provided that fits over the open tops of two adjacently-placed ammunition boxes. A slit in the cover permits the ammunition to pass through the cover and to feed into the guns. Although initial burst can be fired with the cover in place, it is best to remove the cover as soon as feasible when firing is done. The cover is secured in place by hooking the cover grommets on four hooks, which are attached to the support ring.
Spent. Cartridge and Link Chutes
9. Spent cartridges are guided by hoppers and flexible ducts into a container. The empty links are guided into the container through the empty-link chute attached to the cradle-supporting yoke.
10. Two latches for securing the cradle are provided:
one for securing the guns in maximum elevated position, and
0.50-INCH MOUNT, MARK 17 MOD. 1
TRACK AND TRAIN BEARING SECTIONS
0.50-INCH CARRIAGE, MARK 9 MOD. 1-HAND BRAKE ASSEMBLY
another for securing them in a horizontal position. Guns should be secured in a horizontal position on sea when visibility is low. Guns stowed nearly vertically have been mistaken for a submarine periscope.
11. Care. - This mount is a highly developed product and requires to be treated like any other piece of ordnance. Active service and weather conditions at sea will cause salt deposits, rust formation, and oil oxidation; all painted surfaces, therefore, must be kept well preserved and all bearings and working surfaces must be thoroughly lubricated to maintain operation at maximum efficiency.
12. Lubrication. - Grease fittings are provided on the cradle and the support yoke. A grease fitting for the train bearing is located near the hand brake. Turn the rotating structure while forcing in the lubricant so that the grease is uniformly distributed over the entire circle of the roller path-For lubricating train bearing use bearing grease O.S. 1350. This is a special lubricant which was developed to give a maximum of corrosion protection and easy cold-weather operation.
13. Surfaces oscillating or sliding against each other are also to be coated with O.S. 1350 grease or equivalent at assembly or whenever signs of rusting may occur, for examples the hand-brake operating screw, and pin joints of spent cartridge chutes.
14. Train bearing disassembly. -The train roller bearing is packed with grease at assembly and should normally require no further attention except periodic replenishing of O.S. 1350 lubricant. Should any serious trouble develop requiring disassembly and inspection of the roller track, proceed as follows:
(a) Remove support structure from bearing rings by unbolting twelve socket-head screws from support ring. (Hand brake should be in "released" position; cradle with its supporting yoke should be removed for ease and convenience of handling parts.)
(b) Unbolt the two halves of the rotating ring by removing twelve socket-head screws. Remove the upper half of the rotating ring. The rollers may now be removed by tilting the separator sections inward at the top and lifting out the separator with its seven rollers.
15. Train bearing reassembly. - Before reassembly, the train bearing roller track should be thoroughly cleaned and repacked with roller-path lubricant O.S. 1350. Separator strips should be reassembled in raceways in proper position, noting which is the top and which is the bottom edge. Note that one edge of each separator strip is marked "top."
List of Drawings
16. A list of ordnance drawings is given below for the 0".50 Carriage, Mark 9 Mod. 1:
Title of Drawings
Support structure - arrangement
These parts req'd. for mounts with cast yoke socket
Support structure - details - sheet 1
Support structure - details - sheet 2
Support structure - arrangement(alternate)
These parts req'd. for mounts with welded yoke socket
Support structure - details
Support structure - details - sheet 2
Support structure - details - sheet 1
Except pcs. 1 and 7
Support ring - roller track and inner seal details
Train bearing - details
Back rest - general arrangement and details
Pcs. 6, 7 and 8 only
Ammunition box and support - arrangement
Ammunition box and support - details - sheet 1
Ammunition box and support - details - sheet 2
Spent cartridge chute - arrangement and details
Spent cartridge chute flexible tube - arrangement and details
Spent cartridge and link container
Securing latch - arrangement and details
Stowing latch - assembly and details
Ammunition box cover
Ammunition guide bracket (lower) assembly
0".50 CRADLE, MARK 9
1. The 0".50 Cradle, Mark 9 for two 0.50-caliber machine guns, absorbs recoil shock through the utilization of hydraulic shock units. It is a flexible-type cradle, designed to be fired by either or both hands. (See pl. 7.)
2. The cradle frame is made up of four parallel bearers on which are assembled front and rear trunnion slides. Four shock-unit assemblies (two for each gun) are attached at their forward ends to the rear-trunnion slides. At the rear they are attached to the cradle back-plate support.
3. Deflector assemblies are bolted to the bearers and support a yoke and chute assembly serving both guns. Bolts holding outer ends of deflector brackets to bearers also hold continuous-feed rear brackets in place.
4. The cradle back plate houses trigger and safety mechanism, including trigger-synchronizing adjusting screws. This back plate is easily removed by loosening knurled nut on bottom of cradle back-plate support, and the two knurled screws at the top of the hand grips.
5. A sight and stowage-latch support forms the front member of the frame and is designed to accommodate a standard ring sight.
6. Provision is made for mounting a bead sight in a bracket bolted to the yoke and chute assembly.
Functioning of Hydraulic Shock Units
7. The purpose of the hydraulic shock unit is to reduce recoil shock produced by gunfire. This is accomplished by combining the functioning of an hydraulic piston with the reaction of a specially-designed recoil spring.
8. When firing with gun in level position, the major portion of the recoil shock is absorbed by the action of the hydraulic piston - the spring serving to return the gun to normal position after recoil.
9. Firing at a declined angle, recoil is counteracted by the hydraulic piston and weight of gun.
10. With the gun in an inclined position, the spring
offsets the weight of the gun and recoil shock is absorbed by the hydraulic piston.
11. Recoil actuates piston (B1) (see pl. 8, fig. 1) compressing the special recoil spring (B2). This movement develops oil pressure behind the piston, as shown in cut, forcing valve (B3) from its seat, thus permitting oil to flow into the oil reservoir (B4) as indicated by direction of arrows. It will be noted how air ball (B5) is compressed on recoil stroke.
12. On the return stroke (see pl. 8, fig. 2) check-valve
spring (B6) forces valve (B3) closed, and pressure of air in air ball (B5) on oil forces open valve (B7) opening aperture (B8) to facilitate return flow of oil as indicated by the arrows.
13. For guns with flexible-type back plates and spade grips:
(a) Remove gun-back plate.
(b) Remove spade-grip assembly (B9) including back-plate latch lock (B10) and back-plate latch spring (B11) by drilling out rivets (B12). (See pl. 9, fig. 1.)
(c) Remove trigger (B13) , spacer (B14) and spring (B15) by driving out pin (B16). (See pl. 9, fig. 2.)
(d) Insert filler piece (B17) (Part No. 03-038-071) using rivets (B18) (Part No. AN441-6-12). (See pl. 9, fig. 3.)
NOTE: Drilled seat in center of filler (QA-043) must be down to engage back plate latch spring (B21).
(f) Insert trigger assembly (B20) (Part No. 03-038119) and spring (B21) Part No. 03-043-015) ; hold in place by driving back pin (B16). (See pl. 9, fig. 4.)
(g) Replace back plate on gun.
14. For guns with fixed-type back plate.
(a) Remove gun-back plate.
0.50-INCH CRADLE, MARK 9-GENERAL ARRANGEMENT
HYDRAULIC SHOCK UNIT
(b) Remove back-plate filler piece (B22) by driving out back-plate filler-piece pin (B16). (See pl. 9, fig. 5.)
(c) Insert trigger assembly (B20) (Part No. 03-038119) and spring (B21) (Part No. 03-043-015); hold in place by driving back pin (B16). (See pl. 9, fig. 4.)
(d) Replace gun-back plate.
15. The following instructions are applicable to both type guns:
(a) Remove rear trunnion lug (B23) and pin (B24) from rear slide of the cradle and assemble in rear trunnion of gun. (See pl. 10, fig. 1.)
NOTE: No locking device is necessary to hold pin in lug and gun trunnion as position between frame bearers prevent pin movement.
(b) Withdraw pin (B25) (see pl. 10, fig. 2) holding belt-holding pawl (B26) and spring (B27) on gun (feed-opening side) and install guard-plate assembly (B28). Replace pin (B25). (See pl. 9, fig. 3.)
NOTE: When performing this operation, it is necessary to hold belt-holding pawl against pressure of the belt-holding pawl spring, as shown in pl. 10, fig. 2.
(c) Withdraw belt-holding-pawl pin (B25) (link-ejection side). (See pl. 9, fig. 4.) Install link-chute assembly (B29). Replace belt-holding-pawl pin (B25). (See pl. 10, fig. 5.)
NOTE: Operations (b) and (c) are illustrated for right firing guns. For left firing guns, positions of guard-plate assembly (B28), and link-chute assembly (B29) are reversed.
(d) It is important that gun-charging handle be assembled in its most forward position. (See pl. 12, fig. 2.)
Installing Guns in Cradle
16. The machine guns are installed in cradles as follows:
(a) Remove cradle back-plate assembly by loosening knurled screw on under side of cradle back-plate support and the two knurled bolts in the top of the hand grips.
(b) Align threaded lug (B23) on rear-gun trunnion with hole in rear-cradle slide. (See pl. 11, fig. 1.)
(c) Before lowering into position, tilt gun slightly outward to prevent damage to link-chute assembly (B29). (See pl. 11, fig. 2.)
(d) Align front-gun trunnion with drilled holes in cradle front-sliding mount. (See pl.11, fig. 3.) Attaching the following parts:
Rear Trunnion-Lug Nut
Rear Trunnion-Lug Cotter
Front Gun-Trunnion Bolt
Front Gun-Trunnion Nut
Front Gun-Trunnion Cotter
(e) Reassemble cradle back plate. Care should be taken to see that trigger rollers in back plate engage above triggers on gun to insure correct operation.
NOTE: Whenever assembling cradle back plate, it is recommended that knurled screw in under side of back-plate support be drawn up first, and that it properly engages drilled cup in under side of back plate.
(f) To synchronize firing, loosen adjustment-screw lock screws (B30) and adjust trigger-adjusting screws (B31) as shown. After guns are synchronized, tighten lock screws (B30). (See pl. 11, fig. 4.)
NOTE: This adjustment must be made each time guns are installed.
Care and Maintenance
17. With the exception of gun synchronization, described
under "Installing Guns in Cradle," no adjustment of the cradle
INSTALLING GUN IN CRADLE
is necessary. Design of the shock unit compensates for temperature changes and angular-firing positions.
18. Should it become necessary to replenish oil supply in the hydraulic shock units, use hydraulic gear oil, Navy Spec. O.S. 1113 or M-339. To add oil, remove filler plug located at rear of oil reservoir and, using a common oil can, add fluid to overflow level.
NOTE: Shock units must be tipped so that filling-hole face is horizontal when filling.
19. Front slides and chute bearing are fitted with standard pressure-type lubricators (B32) (Part No. AN287-1). These points should be lubricated each time guns are removed for servicing, using bearing grease O.S. 1350. On bearers at rear slide, lubricant should be applied directly to bearers as indicated. (See pl. 12, fig. 1.)
Ordnance Pamphlet No. 951 has been given the following initial distribution:
ComAmphFors - (all)
ComServFor - Pac. Lant, and S W Pac
ComSeaFrons - (all)
Com US Naval Forces, N.W. African Waters
Com Aruba - Curacao
Com Land Craft and Bases, North African Wtrs.
Comdr. all MTB Flotillas and Squadrons*
Comdr. Sub Chaser Dins
Comdr. Greenland Patrol
Comdr. Alaskan Sector
Comdr. Service Squadrons
Comdr. Training Units
Comdr. Escort Div.
Com Fle A/S Warfare Unit
CO MTB Tenders (AGP)
PCO, MTB Tenders (AGP)
Alusna, London only
Commandant Naval District
Commandant Navy Yards
Caribbean Area, Small Craft Project
Naval War College
Post Graduate School
Naval Proving Grounds
Advanced Base Depots
Harbor Entrance Control Posts
Naval Landing Force Equip. Depot
Ft. Schuyler, Ind.
Fleet Amph. Training Sch.
MTB Sqd. Training Center
Naval Firing Range
Construction Training Center
Comdrs, Inshore Patrol
Anti-Aircraft Training Center
Chief of Ordnance - War Department
* Each MTB Squadron to receive 2 copies more than number of craft assigned.
Notification of any desired changes in the above distribution should be forwarded direct to the Bureau of Ordnance. Requests for additional copies of this Ordnance Pamphlet should be addressed to the Commandant and Superintendent, U. S. Naval Gun Factory, Navy Yard, Washington, D.C.; Commandant, Navy Yard, Mare Island, California; or Officer in Charge, Ordnance Publications Subcenter, Naval Supply Depot, Pearl Harbor, Hawaii, U.S.A.