2008 October 4 update: Larger gridded gun image added.
2008 September 24 update: Gridded HB barrel and carrier added.
2008 September 23 update: Image of heavy-barrel and water-cooled guns, overlaid with a grid, added.
2008 August 14 update: Page layout change, reorganization and additions. History text moved here.
The following images show dimensions for the Browning .50 caliber aircraft machine gun as used on WW2 PT boats. These dimensional images represent a best effort on my part, please confirm dimensions and information given here for yourself if errors cannot be tolerated. Images are for non-commercial personal use only, please obtain permission for any other use.
The main references used were side plate, barrel jacket, and trunnion adapter drawings which gave a solid dimensional base along with knowing the overall length. Being closely flanked by the ammo guides and mount, PT images often have gun details obscured. The guns in turn hide parts of the mount. Wartime military manuals and part images helped fill the gaps. Please let me know if you notice any errors. Some other online information sources:
http://www.ibiblio.org/hyperwar/USN/ref/MG/index.html The Machine Gun. History, Evolution, and Development of Manual, Automatic, and Airborne Repeating Weapons by George M. Chinn, Lieutenant Colonel, USMC
Gary's U.S. Infantry Weapons Reference Guide - .50 cal.
Springfield Armory National Historic Site
In the U.S., actual guns can only be owned by special license. Maybe a good thing if you consider your idiot neighbor being able to buy one at a pawn shop, and that being inside of a brick building is no protection against one. Not to mention that a misadjusted gun can explode with lethal force since chamber pressure is about 50,000 pounds. As far as I know, only guns already manufactured and registered by about mid 1986 can be owned. Even a long lost old classic is not registrable although certain official agencies and museums are able to take ownership. Even the cutaway training guns have to be registered. In addition, states may have their own laws. For obvious reasons, besides being unnecessary for modelers, the plans shown here do not include any of the internal details to attempt to make a working version.
In 1930 the Browning .50 caliber water-cooled M1921A machine gun was standardized. While satisfactory for its intended use as an antiaircraft gun, there was also a need for lighter, more flexible guns not attached to a 40lb jug of water. The Browning .50 caliber M2 machine gun, put into service in 1933, was the answer. It was designed for three different basic configurations. They are the aircraft (AC), heavy-barrel (the HB, still in use today), and water-cooled (WC). They all use the same operating mechanism which was a great advantage in their manufacturing and service. Another feature of the M2 is the flexibility to feed ammunition from the left or right, a requirement of the Air Corps for its aircraft installations. Various assemblies to accommodate a particular mount and usage include an operating slide with a rear mounted charging handle, back plate grips and trigger, a side plate trigger, solenoid triggers, feedway and short round protecters, sights, and heaters. Mounts usually included the correct trigger.
The aircraft gun is easily identified by its shorter barrel and perforated jacket. Because of its shorter, lighter barrel the AC gun has a higher rate of fire although the trade-off is that velocity is reduced.
The basic aircraft gun can be a fixed or flexible type with a few simple part changes. Fixed means that the gun is solidly mounted such as in a wing installation, flexible means that the gun is operator controlled with the mount free to swing up / down / left / right.
The right hand side plate of an M2 is engraved with the guns information, the left is blank.
The M2 is accurate out to about 2,000 yards, so accurate that it saw occasional use as a sniper rifle.
Although the belt is described as a "disintegrating link" type, the links don't actually disintegrate. The term refers to the belt appearing to disintegrate. Perhaps a better choice would have been "disassembling link" since the guns link stripper holds the link while the extractor slides the cartridge out, leaving the links whole and in pretty much the same shape as they were.
The belt is loaded into the gun with the double end of a link first. Spent links get pushed out the side opposite of the feed, spent cases exit out of the bottom of the gun.
Browning .50 cal. M2 manual designations
The gun went through several official designations and also has a few nicknames. By the end of the war it was standardized as a "Browning Machine Gun, Caliber .50, M2". See the image at right for the some of the names found in a 1946 list of Naval ordnance manuals. The gun is also known by several slang terms such as a BAM(G) for Browning Aircraft Machine Gun, the BMG for Browning Machine Gun (current for the HB), and "Ma Deuce" from the M2 designation.
United States World War II manufacturers, Browning Machine Gun, Caliber .50, M2
- Colt's Patent Firearms Co.
- High Standard Co.
- Savage Arms Corp.
- Buffalo Arms Corp.
- AC Spark Plug
- Brown Lipe-Chappin
- Saginaw Divisions of General Motors Corp.
- Kelsey Hayes Wheel Co.
The basic aircraft gun came without a trigger or the side bracket with the charging handle called the retracting slide group (RSG). PT boats used this basic gun along with a mount supplied trigger and usually a simple bolt handle for charging the gun instead of the RSG. Unlike the RSG, which operates against a short stud and remains stationary during firing, the bolt handle travels back and forth with the bolt.
PT Flash Hiders / Supressors
Various flash hiders (at the end of the barrel) were occasionally used, and were mounted with an adapter that replaced the normal barrel end bushing in the jacket. A canister style seems to be most prevalent on earlier boats (PT's 61, 108, 109, 141). I have images of a few later boats (PT's 363, 519), and PT 59, with a large cone shaped hider, and PT 588 with a smaller cone type.
Most of the steel gun parts have a parkerized finish, even the cotter pins and springs. This left the metal a matte dark charcoal gray/black color when fresh. Modern parkerizing appears lighter. The coating has a porosity that will hold a layer of corrosion preventing oil. I've lightened the colors on the plans images for better detail visibility. The auxiliary charging handle appears bare metal silver in PT images although an image found below shows one parkerized. In general, the color of parkerized finishes is debated and can vary widely depending on the type of formula used, how it is applied, steel alloy, and age/wear. Color can range from a well-oiled glossy black to light greenish gray. Vintage parts stored in cosmoline can turn a dark green. Here is a good article about the parkerizing process.
Found below the images are details from a PT boat manual, a list of older BMG related manuals, and a list of some of the changes in the early guns.
New and old bolt handle styles:
Note retracting slide lever studs in different holes than normal:
Different types of attachments:
PT Manual Excerpts
Selected text from the Navy PT boat tactical and doctrine manual found at http://hnsa.org/doc/pt/doctrine/index.htm (lent for OCR scanning by ptboats.org):
HEADQUARTERS OF THE COMMANDER IN CHIEF UNITED STATES FLEET
MOTOR TORPEDO BOATS
TACTICAL ORDERS AND DOCTRINE
Part 1. General Doctrine / Chapter 1. General and Military Characteristics / Military Characteristics
1107 (4) PT 103 type-PTs 103-196 (Elco Boat Works). Length 80 feet 3 inches; beam 20 feet 10 3/4 inches; max. draft 5 feet 3/4 inch; displacement 100,000 pounds.
Four .50-caliber air-cooled B. A. machine guns, two twin, hand-operated scarf ring mounts.
Allowance-10,000 rounds per gun.
Carried on board-1,000 rounds per gun belted (ratio one Tr. to one A. P.) in four 250-round magazines per gun.
Part 4. Armament-Doctrines and Standard Procedures / Chapter 2. Gunnery Orders and Standard Procedure
4201. Personnel, training and safety precautions.-Each member of a MTB crew must be thoroughly familiar with and capable of operating all armament on board. Training toward higher standards of personnel operating efficiency should be continuous. Safety orders will be posted in conspicuous places near the ordnance equipment concerned. These safety orders must be rigidly observed at all times.
4202. Care and upkeep of weapons.-The gun armament on MTB's consists of rapid fire machine guns which require meticulous care and constant routine maintenance to keep them in proper condition for effective use. They are exposed to large quantities of salt spray and even under most favorable conditions will receive rough treatment. Every effort should be made to keep all guns in a high state of readiness. In this connection light canvas muzzle bags should be fitted on guns when it is necessary to keep them trained on bearings where salt water is likely to fill up the bores. In case of emergency the gun can be fired with muzzle bags on.
4203. Fire control.-Gun control in MTB's is primarily a matter of firing the maximum number of guns at enemy targets within machine gun range. The .50-caliber and 20-mm. machine guns are short range weapons with a very high rate of fire. Their fire must be held until the target comes within their range. Indiscriminate firing at distant targets must not be permitted. The ammunition for these guns is expended very rapidly, only a limited amount can be carried on board, it is precious and for use at the proper time. These guns are for defense against dive bombers, strafers, low position glide bombers, and for offense or defense against small surface craft, large surface craft (during close in or night attacks) and submarines on the surface.
4204. Fire distribution.-No standard fire distribution plans are necessary. During night torpedo attacks MTB's will seldom, if ever, be in formation. During daylight torpedo attacks it is doubtful if the attack can be pressed in to machine gun range. Against aircraft, the nearest and most menacing target should be engaged and concentrated on.
4205. Surface targets.-If surface craft are engaged by MTB's in formation, all enemy ships being attacked must be kept under fire. Whenever the number of torpedo boats exceeds the number of enemy ships, the fire will be distributed as evenly as possible. Crossfire should be avoided. Maneuvers should be made in a manner that will not mask the fire of any boat, but if, possible to mask some of the enemy's guns.
4206. Aircraft targets.-When engaging enemy aircraft, maneuvers should be made in a manner that will afford mutual fire support and avoiding enfilade. (See art. 3903.)
4207. Directing fire.-When engaging enemy surface craft, fire should be directed at the bridge, control stations, exposed gun crews and (if at night) searchlights. Against a submarine, it is possible with well directed machine gun fire to prevent the submarine from manning his guns, and either force him to surrender, or to submerge and lose the men on deck.
4208. Firing at night.-During night or low visibility an MTB should never open fire against a surface ship unless it is certain that the MTB has been sighted or that the surface ship itself is poorly armed. During daylight high visibility, it can be assumed that an MTB has been sighted by the time the enemy is within gun range.
4209. Orders for opening fire.-When in formation boats should follow the movements of the leader in opening fire. However, any boat observing aircraft actually attacking should fire a short burst in the direction of the attack if it is believed the other boats have not seen the plane or planes. Vessels observing enemy surface craft or distant aircraft which it is believed the leader has not seen, should pass the information by visual signal or radio. Orders to open and cease fire will always be given from the conning station and must be strictly obeyed.
4210. Opening fire ranges.-The maximum opening range for 20-mm. and .50-caliber machine guns should be 2,000 yards. This is the "tracer burn out" range for both weapons and is also the limit of their accuracy. When firing at aircraft, opening fire ranges considerably less than 2,000 yards should be used in order that fire will not have to cease due to an expended magazine in the case of the 20-mm. and an overheated gun in the case of the .50-caliber. Recommended ranges for opening fire are:
Dive bomber-1,500 to 2,000 yards.
Strafer, low position glide bomber or torpedo plane-1,000 to 1,500 yards.
NOTE.-MTB's can be used to a great advantage in protecting large vessels against surprise torpedo plane attacks by taking station at approximate points of torpedo release thus subjecting these planes to much additional gunfire.
4211. Firing on attacking aircraft.-If planes are attacking in rapid succession, fire should never be continued on planes which have passed overhead. It should always be directed at incoming planes. The principle of engaging the nearest and most dangerous targets applies.
4212. Lookouts.-The importance of keeping an extremely bright lookout in all directions cannot be overstressed. Each man having a battle station above deck should be assigned a search sector. Dive bombers invariably attack from the direction of the sun. The lookout assigned that sector should wear dark glasses. MTB engines are as loud as those of an airplane. This factor may enable a plane to attack a boat that is "not alert" before any gun on the boat can be brought to bear.
4213. Standard terms.-Definitions of terms used in control of machine gun fire are as follows:
(a) "Track" is an order given before "commence firing" and means bring the guns into firing position and follow target.
(b) "Tracking" is a general term meaning the art of moving the gun smoothly and steadily keeping the sights or tracers constantly on the target.
(c) "Individual tracer control" is a method of fire in which the gunner plays the tracer stream on the target in same manner as he would water from a hose.
(d) "Tracer control" is a method of firing in which the gunner uses tracers at the target, to assist in tracking.
(e) "Sight control" is the method of firing in which the gunner uses sights in tracking the target.
(f) "Tracer cut back or tracer curve illusion" is the apparent curve in the path of tracers as they pass a moving target. This is due to the relative movement between target and tracer.
(g) "Superelervation" is the added angle of elevation necessitated by a projectile's trajectory. It is the angle between a line from gun muzzle to any given point in space and the axis of the gun when laid so that projectiles will hit the given point.
(h) "Slant range" is the range along the line of sight from gun muzzle to the target.
4214. Individual tracer control will normally be the method used in firing machine guns on MTB's. Using this method, the gunner must not sight along the gun barrel while firing because in so doing, he is unable to observe tracers correctly. He must assume a relaxed position behind the gun and focus his eyes on the target. With his eyes so focused, he is best able to distinguish where the tracers are going in relation to the target. When hitting, tracers appear to be going through target in a curved line (tracer cut back). Do not attempt to follow tracers from gun. Concentrate on area around target. The gunner must track smoothly, always moving gun in direction of target course, never toward the rear of the target. If he is ahead of the target, he should slow his rate of track until the target enters the tracer stream, then resume correct rate. When behind, make a bold correction to bring tracers ahead of target.
4215. Tracer control wherein the gunner aims with sights but the tracers are observed at the target to assist in tracking and hitting. This method will be used:
(a) With 20-mm. fire when the Mark 14 gunsight is installed.
(b) If the ratio of tracers becomes less than one in five, due to scarcity of tracer ammunition.
4216. Mark 14 gunsight.-The Mark 14 gunsight is a gyro lead-computing sight which greatly increases the accuracy of 20-mm. fire directed at rapidly moving targets. The sight is mounted directly on the gun, and requires no stabilized reference position or connections to the gun trunnion or ship's deck. The equipment is of the optical reflex type in which the line of sight, as indicated by an illuminated recticle, is displaced from parallelism with the gun bore. The angular displacement, or lead angle, corrects for the motion of the target during the flight of the projectile and introduces the necessary superelevation. The gunner swings the gun so as to hold the illuminated reticle on the target and another member of the gun crew makes the required adjustments for range effects by means of a manual control. This is usually done by first setting an estimated range and then introducing spots as prove necessary from tracer observation.
If tracers become less than one in five, ring sights should be installed on guns and tracer control used. All personnel should know how to aim using ring sights. The attitude and speed of the target determines the aim-off necessary. The speed rings indicate the amount of lead to be given for the target speed component across the line of sight and not the lead for the actual target speed. The tracers should be observed at the target to aid in determining the correct aim-off. "Smooth tracking is essential to tracer control."
4217. Sight control becomes necessary if no tracers are available. The discussion of tracer control applies except that there are no tracers to observe passing the target.
4218. Conditions of readiness.-For the condition of readiness of the gun battery see paragraph ___.
4219. Routines-Instruction books.-Routine upkeep, maintenance and overhaul forms will be provided and rigidly adhered to. The following publications should be used as guides and references in connection with the gun battery:
MTB familiarization pamphlet.
Bureau of Ordnance Manual.
O.P. 813-20-mm. A.A. gun and mount.
T.M. 9-12-25 -50-cal. B. A. M.
T.M. 9-12-26 -50-cal. B. A. M.
O.P. 529 -.45-cal. Thompson submachine gun.
O.P. 68 -.45-cal. Colt automatic pistol.
O.P. 595 -.30-cal. Springfield rifle.
O.P. 596 -.30-cal. Springfield rifle.
Part 6. Useful Information and Data / Chapter 1. (A) Conditions of Readiness of Armament
6107. .50-caliber gun-Ready condition.
1. Guns clean and well oiled.
2. Oil buffers filled.
3. Head space set.
4. One thousand rounds up in each turret well cleaned and free from corrosion; not led up to gun.
5. Spare magazines loaded and at hand in tank room.
6. Waterproof cover on and secured only in wet weather. Normally off during daylight. Covers on, not secured at night.
6108. .50-caliber gun-Fully ready condition.
1. Lead belt up to gun, half load, put on safety.
2. If weather is wet, keep muzzle bags on guns, cover over turret.
3. Guns manned, trained on bow or probable target bearing.
6116. .50-caliber B. A. gun data (air cooled).
1. Rate of fire-500-650 rounds/minute-adjustable.
Muzzle velocity-(78 feet from muzzle).
Cartridges caliber-.50 M1-2,669 f/s.
Cartridges caliber-.50 M1,823-2,500 f/s.
Tracer is loaded to group with ball and A. P. at 1,000 yards.
2. Maximum range 7,125 yards at 642.3 mils elevation. Time of flight 39.7 seconds. Drift 6.7 mils.
3. Time of flight.
500 yards 072 seconds.
1,000 yards 1.45 seconds.
1,500 yards 2.40 seconds.
4. Number of rounds first burst and subsequent bursts. An initial burst of 75 rounds may be fired, after which 20 rounds a minute may be fired. After waiting 15 minutes without firing, another 75 round burst may be fired. If an initial burst of only 25 rounds is fired, 25 rounds a minute may be fired continuously. The above rate of fire prevents overheating the gun, impairing its accuracy, and reducing its life. However, in combat the gun may and should be fired without regard to this.
Here are the titles (exactness not checked), and the oldest date I could find, for some early .50 manuals to show some of the various types:
TM 9-1005-209-50 MULITIPLE CALIBER .50 MACHINE GUN, TRAILER MOUNT M55 COMPOSED OF MULTIPLE CALIBER .50 MACHINE GUN MOUNT M45C AND 1-TON, 2 WHEEL MACHINE GUN MOUNT TRAILER M20 AND MULTIPLE CALIBER .50 MACHINE GUN MOUNTS M45D AND M45F 1958
TM 9-1005-213-10 OPERATORS MANUAL, MACHINE GUN, CALIBER .50 BROWNING, M2, HEAVY BARREL, FLEXIBLE, W/E; MOUNT, TRIPOD, MACHINE GUN CALIBER .50, M3, W/E; MOUNT, MACHINE GUN, ANTIAIRCRAFT CALIBER .50, M63, W/E 1968
TM 9-1005-231-10 OPERATORS MANUAL, MACHINE GUN, CALIBER .50, FIXED, M85 1974
TM 9-1005-245-14 OPERATORS AND ORGANIZATIONAL MAINTENANCE MANUAL, DIRECT SUPPORT AND GENERAL SUPPORT MAINTENANCE MANUAL INCLUDING REPAIR PARTS, AND SPECIAL TOOL LISTS (INCLUDING DEPOT MAINTENANCE REPAIR PARTS AND SPECIAL TOOLS). VARIOUS MACHINE GUN MOUNTS AND COMBINATIONS USED ON TACTICAL AND ARMORED VEHICLES 1973
TM 9-1223 TECHNICAL MANUAL, TWIN CALIBER .50 MACHINE GUN MOUNT M33 AND MULTIPLE CALIBER .50 MACHINE GUN MOUNT M45, TWIN AND QUAD FIFTY ORDNANCE MAINTENANCE MANUAL, TWIN CALIBER .50 MACHINE GUN MOUNT M33 AND MULTIPLE CALIBER .50 MACHINE GUN MOUNT M45 1944
TM 9-1225 TECHNICAL MANUAL, BROWNING MACHINE GUN, CALIBER .50, ALL TYPES 1942
TM 9-219 TECHNICAL MANUAL, BASIC AIRCRAFT MACHINE GUNS, CALIBER .50, T36 and T25E3 (M3) 1945
TM 9-222 TECHNICAL MANUAL, MULTIPLE CALIBER .50 MACHINE GUN MOUNT M45, QUAD FIFTY TECHNICAL MANUAL, MULTIPLE CALIBER .50 MACHINE GUN MOUNT M45 1943
TM 9-223 TECHNICAL MANUAL, TWIN CALIBER .50 MACHINE GUN MOUNT M33 AND MULTIPLE CALIBER .50 MACHINE GUN MOUNT M45, TWIN AND QUAD FIFTY TECHNICAL MANUAL. TWIN CALIBER .50 MACHINE GUN MOUNT M33 AND MULTIPLE CALIBER .50 MACHINE GUN MOUNT M45 1944
TM 9-225 TECHNICAL MANUAL, BROWNING MACHINE GUN CALIBER .50, M2 AIRCRAFT, BASIC 1940
TM 9-225 TECHNICAL MANUAL, BROWNING MACHINE GUN CALIBER .50, M2 AIRCRAFT, FIXED AND FLEXIBLE 1942
TM 9-225 TECHNICAL MANUAL, BROWNING MACHINE GUN CALIBER .50, AN-M2 AIRCRAFT, BASIC 1947
TM 9-225A TECHNICAL MANUAL, BROWNING MACHINE GUN CALIBER .50, M2 AIRCRAFT, FIXED AND FLEXIBLE 1940
TM 9-226 TECHNICAL MANUAL, BROWNING MACHINE GUN, CALIBER .50, M2, WATER-COOLED, AND MOUNTS 1940
TM 9-230 TECHNICAL MANUAL, MACHINE GUN MOUNTS FOR BOATS 1943
TM 9-789 MANUAL, TRAILER, MOUNT M20 (COMPONENT OF MOUNT, TRAILER, MULTIPLE CALIBER .50 MACHINE GUN, M55), QUAD MOUNT TECHNICAL MANUAL, TRAILER, MOUNT M20 (COMPONENT OF MOUNT, TRAILER, MULTIPLE CALIBER .50 MACHINE GUN, M55) 1944
TR 1300-50A INFANTRY AND AIRCRAFT ARMAMENT. BROWNING MACHINE GUN, CALIBER .50, M1921, WATER-COOLED 1932
TR 1300-50B INFANTRY AND AIRCRAFT ARMAMENT. BROWNING AIRCRAFT MACHINE GUN, CALIBER .50, M1921 1931
FM 23-60 BASIC FIELD MANUAL, BROWNING MACHINE GUN, CALIBER .50, HB, M2 GROUND 1940
FM 23-65 ARMY FIELD MANUAL, BROWNING MACHINE GUN CALIBER .50 HB M2 1944
FM 4-155 ANTIAIRCRAFT ARTILLERY FIELD MANUAL, SERVICE OF THE PIECE CALIBER .50 AA MACHINE GUN 1943
FM 44-57 MANUAL, MULTIPLE MACHINE GUN MOUNTS, QUAD FIFTY FIELD MANUAL, SERVICE OF THE PIECE MULTIPLE MACHINE GUN MOUNTS 1945
ORD 8 SNL A-38 MANUAL, FIELD AND DEPOT MAINTENANCE ALLOWANCES FOR GUN, MACHINE, CALIBER .50, BROWNING, AN-M2, AIRCRAFT, BASIC 1951
OPNAV 33-40/NAVAER 00-80S-40 AIRCREWMAN'S GUNNERY MANUAL 1944
For a .30 to show similarities:
TM 9-205 TECHNICAL MANUAL, BROWNING MACHINE GUN, CALIBER .30, M2, AIRCRAFT, FIXED AND FLEXIBLE 1944
Major changes in the M2 have been few since the gun is designed to be adaptable to different configurations using the same basic parts. Although I'm sure there are other design changes besides the following, here are some I've noticed or read about:
Please let me know any others or correct me if you find something wrong.
The left side plate trigger bar pin was changed from a flat piece kept in place by a protrusion into a side plate hole, to several pin types retained by a spring-loaded tab held in place by the top plate.
Some right side plate feed brackets have an extension off the back and some don't. A 1944 air gunnery manual shows a feed cycle sequence using a top view of a real gun with only the last gun in the sequence having an extension on the right side. It also has a retracting slide while most others don't. A manual states that the bracket is an early style and includes instructions to trim it off. I have an image of a PT .50 with the extension.
Some left side feed brackets have a dished cutout in the rear ear.
Very early guns did not use the right-hand rear cartridge stop assembly (used with L. H. feed), they used a separate rear cartridge stop and link stripper like the left side has when set for R. H. feed. A January 1943 manual shows the assembly being used. The combo stop was more than just a replacement for the separate pieces. It also had a spring loaded pawl that caught the case extracting groove to help align it, and served as a short-round eliminator.
Early guns use a single belt holding pawl with a single spring. Because of occasional stoppages due to heavy belt loads (the front of the cartridge would swivel out), this was modified to use dual springs then later changed to a split type pawl. A 1944 manual states the dual pawls started being used on P-51 Mustang guns with notes on converting older guns to use them. Aircraft could have exceptionally high feed loads caused by long belt routing, gravity force change through banking, and g-forces added to the pull that the guns feed mechanism needed to exert. PT boat guns also had extra high loads due to the fixed ammo boxes being so low. The ammo box is fixed to the gun cradle on most M2 mounts and maintains position relative to the gun. Jerks from wave slap must also have been a factor.
A spec of 17 pounds of lift for the feed mechanism was later increased to 30lbs in the short lived 1944 T36 / M2A1 gun. About 8,000 guns, out of a contract for 31,336, were made before the order was cancelled in favor of the 1945 M3.
The pin assembly holding the feed parts can be straight with a small knob at the end or with a 90 (+) degree bend. They are held in place with either a cotter pin or have a slot machined for a spring pin.
Early guns with the retracting slide group may use only one bracket bolt.
The early retracting slide lever stud was found to be weak and was replaced by one with a larger diameter thread. Early slides were drilled and tapped to accept the new stud.
There was a Special Cover Latch B7161236 for installations where access to the side lever might be restricted. It replaced the latch in the center rear of the cover and had an arm that came up and arced back so a finger could lift and release the latch.
Retracting slide brackets have 1, 2, or no top / bottom cutouts, some have ribs along the width of the center slot, and early types can be a lighter design with a reduced thickness in the forward part. 1943 and 1944 manuals show the lighter style with a single cutout for the trigger bar pin when mounted on the left side of the gun.
Several wood and plastic retracting slide grips are used. The early ones are fastened with a long screw through the grips. The threaded end of the shaft is split and drilled for a taper pin to lock it against the lever threads. Because of grip bolt breakage problems, later guns use an assembly with a stud that screws into the lever. A 1944 manual mentions the new grip assembly as being standardized.
The weld filet between the barrel jacket D28255 and breech bearing B8921 was large and had to be ground down when used with Air Corps adapters E-8, E-10, or E-12. The weld was fine on most other mounts. As manufactured, the weld filled about a 30 degree angle from the outer diameter of the breech bearing flange, to the surface of the jacket with about a .25" radius at the jacket. The flange is the larger diameter round ring by the base of the jacket with wrench notches. To use with the adapters mentioned, the weld was reduced to a radius of no more than 3/32".
Back plates varied including a vertical buffer model that was widely used on fixed guns prior to 1939.
The right hand side plate of an M2 is engraved with the guns information, the left is blank. According to a 1944 manual, High Standard manufactured guns 151125 through 162799 were erroneously marked on the right hand side plate:
Colt Aircraft Machine Gun
Model MG 53-2 Caliber .50
and the ordnance department was instructed to X out all but:
XXXX XXXXXXXX Machine Gun
XXXXX XX XXXX Caliber .50 M2
Note the added "M2". In the 1944 manual the gun was on record as "Gun, Machine, Cal .50, Browning, M2, Aircraft, Basic".
High Standard guns 151125 through 159024 had an additional issue, the trigger bar pin hole in the top plate bracket wasn't finish reamed and a stud shoulder was mispositioned. This caused an assembly problem with flexible guns although fixed were OK.
The bottom plate lug on the breech lock cam was widened in 1940, and the corresponding cutout in the bottom plate enlarged from .625" X .763" to 1.533" X .763". The bottom plate cutout is under the "upside down" castle nut.