2010 March 5 update: Added excerpts from the manual 0.50-Inch Machine Gun Mounts, Mark 17 Mods. 1 and 2. Corrected the minimum elevation of the mount from 5 degrees to 15 degrees. The 5 degree lower limit is via the lower bar on the Elco 80' depression rails.

2010 January 6 update: Jerry Gilmartin supplied Richard Pekelney of hnsa.org with a turret and .50 mount manual. Richard scanned the manuals and added them to the documents page. They are the 0.50-Inch Machine Gun Mounts, Mark 17 Mods. 1 and 2, 1943, and The Dewandre-Elco Power-Operated Machine Gun Turret used by earlier PT boats.

2009 October 1 update: 5 view of Mark 9 carriage added. Jerry Gilmartin supplied me with some measurements from an actual carriage on PT 658, thanks Jerry!

2009 September 1 update: 5 view of Mark 9 cradle layout changed and elevations added.

2009 August 20 update: 5 view of Mark 9 cradle added. Back plate retaining screw added to part details drawing (missing on details and exploded views).

2009 April 22 update: Al Ross, author and PT expert (http://www.coastalforcesplans.com/), let me know some facts about this mount (which is actually just the cradle) when I asked at the ptboats.org forum about a reference image of an I.D. plate I have for the cradle that showed it as a "Mark 9":

Actually, the plate shown is for a MK9 cradle, not a MK9 mount. This is consistent with the components of a MK17 Mount, which, according to the MK17 manual (OP 951), included the Carriage, MK9, Cradle, MK9, and Sight, MK11.

Also at the forum, Gene Kirkland (http://geocities.com/pt_king/PTboats.html), posted a link to a new PT manual on the net that has a section on this mount:
http://www.ibiblio.org/hyperwar/USN/ref/PT-Manual/index.html

Finally, I updated the drawing after working on a full scale version for Jerry Gilmartin, curator of PT 658.

2009 January 23 update: Page added.

Although the 107 fresh from the factory, as seen in an image here, shows attached accordion style shell chutes and a link guide leading from the bottom of the mount to the catch bin built into the turrets rotating ring, none of the other images I have show them. But they can be hard to see so I'm not sure if they were usually removed. I'll update if / when I get better references.

From the movie "They Were Expendable"

Excerpts from the manual 0.50-Inch Machine Gun Mounts, Mark 17 Mods. 1 and 2:

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 MG 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.

Chapter 1
GENERAL DESCRIPTION

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:

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).

Weights
Weight of the Mount - without guns, ammunition or protective shield: 495 lbs.
Weight of two guns: 130 lbs.
Total: 625 lbs.
Weight of 1000 rounds of ammunition in links: 300 lbs.

Chapter II
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.

Train Bearing
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.

Hand Brake
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.

Cradle-Locking Latches
10. Two latches for securing the cradle are provided: one for securing the guns in maximum elevated position, and 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.

Chapter III
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.