Aircraft technical Basics: TM 1-409, Aircraft Armament and Pyrotechnics, 1941: VI. - Aircraft Machine Guns, Caliber .30

Section VI - AIRCRAFT MACHINE GUNS, CALIBER .30

36. General.-a. Definition.-A machine gun is an automatic weapon which is self-operative, within the limits of the. capacity of the ammunition belt, by simply keeping the trigger depressed. It incorporates within its own mechanism the functions of loading, firing, extraction, and ejection of the spent ammunition.

b. Uses.-Aircraft machine guns are mounted on airplanes for use offensively and defensively against enemy aircraft and ground objectives.

c. Models.-The Army Air Corps has, at present, two types of machine guns. They are the

(1) Caliber .30 machine gun.

(2) Caliber .50 machine gun.

The term "caliber" is used to denote the diameter of the bore of the barrel in inches.

d. Types.-By changing some of the components of the caliber .30 or caliber .50 machine guns, these guns can be mounted for use as either fixed or flexible weapons.

(1) Fixed gun.-A fixed gun is mounted rigidly to some structural member of the airplane and fires approximately parallel to the direction of flight. In fixed guns, aiming of the weapon is accomplished by maneuvering the airplane while in flight; the sights mounted on the airplane having been previously adjusted so that the projectile will intersect the line of sight at a predetermined distance. Since single-seater pursuit airplanes and other small type airplanes are highly maneuverable, they are quite adaptable to a fixed type of gun installation. Further, it would be impracticable for a pursuit pilot to operate a flexible gun and operate the airplane at the same time.

(a) If the gun is mounted adjacent to the engine, it will be necessary to fire through the plane of propeller rotation. In such installations the gun fire is controlled so as to miss the propeller blades by action of the synchronizing mechanism, which is a device operating between the engine and the gun for this purpose.

(b) If the gun is mounted in such a position that the line of fire is outside the plane of propeller rotation, no controlling device is necessary to insure that the projectiles will not strike the propeller blades.

(2) Flexible gun.-The flexible gun is installed in the fuselage of the airplane and is provided with a backplate having double spade grips and a hand trigger. The gun is mounted on a flexible mount which permits the aerial gunner to swing the gun about, both horizontally and vertically, thus enabling him to create a larger field of fire than if the gun were fixed.

37. Caliber .30 Browning aircraft machine gun, M2.-a. Type of operation.-The standard caliber .30 aircraft machine gun for use in the Air Corps is the Browning caliber .30, M2. It is a recoil-operated, belt-fed, air-cooled machine gun.

b. Principle of operation.-Recoil is the reactionary force to the burning of the propellent charge within the barrel. In recoil operated guns, the force of recoil is utilized to drive the mechanisms to the rear. The action is driven back forward by springs contained within the gun.

38. Ammunition belts. a. The ammunition is assembled into metallic belt links and may be formed into any desired length of belt. The belts must be free from defective links, kinks, or tight hinge loops, so that cartridge heads may be well alined, and the belt will never be oiled, since it will collect grit and cause damage to the gun.

b. Loading an ammunition belt in the gun may be performed with the cover either open or closed. In either case, always enter the double loop end of the belt through the belt feed opening until the first cartridge is beyond the belt holding pawl. Close cover, if open. Pull bolt completely to the rear by means of the retracting slide (flexible gun) or operating slide ( fixed gun) and release it.

39. Stoppages.-a. Definition.-A stoppage results from a malfunction due to improper or incomplete action of some part of the gun or its ammunition ; for example, failure to extract. the empty cartridge case.

(1) Any accidental cessation of fire is a stoppage. It may be a faulty cartridge or a malfunction of some part of the gun. The fact that all the ammunition in the belt has been exhausted should not be called a malfunction, since it is a cessation of fire from a natural result.

(2) Proper care of the gun and attention to the points before and after firing will greatly reduce the liability of stoppages, particularly if the gunner has an intelligent understanding of the reasons why stoppages occur.

b. Classes of stoppage.-Stoppages may be classed under two main headings :

(1) Temporary.-Temporary stoppages are those stoppages which can be corrected by immediate action to the gun and while the airplane is in flight.

(2) Prolonged.-Prolonged stoppages are caused by a failure of some part that, as a rule, cannot be remedied without skilled assistance. These necessarily put the gun out of action for a more or less prolonged period of time.

c. Immediate action.-Immediate action is the immediate application of a procedure designed to remedy the stoppage and put the gun in action in the shortest possible time. Drawing the bolt to the rear by means of the charging handle, then releasing it, constitutes "immediate action" for an aircraft machine gun.

d. Classification of stoppages.-Stoppages are classified according to the position in which the bolt is found when the gun stops firing. The position of stoppage is an indication as to the particular malfunction that caused the gun to stop. Therefore, it is of vital importance to the armorer that he know as much as possible about the causes of and remedies for the various stoppages, so that he may subsequently apply this knowledge to quickly determine whether any particular stoppage can be readily corrected or if it requires more extensive repair than he can make.

40. Care and preservation-a. General.-The importance of a thorough knowledge of how to care for and clean the machine gun cannot be overemphasized. The kind of attention given to a weapon of this type determines largely whether or not it will shoot accurately and function properly when needed. The bore and the chamber must be kept in perfect condition for accurate shooting. Also, it is just as important that the receiver and moving parts be kept clean, lubricated, and in perfect condition for efficient functioning.

b. Points to be observed before flight.-The following points must be observed before an airplane goes on a gunnery mission in order to detect and correct troubles which would probably cause malfunctions during firing :

(1) Wipe the bore and the chamber of the barrel.

(2) See that the adjusting screws are screwed in tightly against the buffer disks in the backplate.

(3) Test functioning of gun by hand operation of the bolt.

(4) Oil carefully.

(5) Make sure that the ammunition belt is in good condition and loaded correctly. Defective belts will not be used since they will cause stoppages.

(6) Carefully place the belt of ammunition in the box and check box for proper alinement.

c. Points to be observed after flight.-The following points must be observed as soon after a gunnery mission as practicable :

(1) Unload the gun completely and remove the belt, if any.

(2) Clean the bore and all working parts. If this cannot be done at once, oil carefully to prevent rust.

(3) Release the firing pin.

(4) At the first opportunity, dismount the gun; clean, oil, make needed repairs and replacements and adjustments.

(5) The armorer must get a detailed account from the gunner or pilot of the gun's behavior in the air. If stoppages have occurred, their cause must be determined and corrected immediately in order that the gun will function correctly on the next mission.

d. Care and cleaning of gun.-(1) In cleaning the bore

(a) Disassemble the parts from the gun.

(b) Place the barrel, muzzle down, in a vessel containing hot water and issue soap, or, lacking these, hot water alone.

(c) Insert cleaning rod, with a flannel patch attached, in the breech end. Move the rod forward and backward for about 1 minute, pumping water in and out of the bore.

(d) Use a brass or bronze wire brush while the bore is wet, running it forward and backward through the barrel three or four times. Never use abrasives such as steel wool which will wear down the lands in the barrel and render it unserviceable.

(e) Pump water through the bore again to clean it.

(f) Remove the barrel from the water, and using a dry clean patch, thoroughly swab the bore until it is perfectly dry and clean.

(g) Saturate a patch with oil and swab the bore again. Allow a thin film of oil to remain in the bore. Oil used will be that specified by Technical Orders; sperm oil will never be used as a gun lubricant.

(h) Guns should be cleaned not later than the evening of the day on which the gun is fired, preferably immediately after cessation of firing.

(i) Inspect and clean guns for 3 days following cessation of firing.

Note.-The most recent cleaner specified by the Ordnance Department for cleaning the bores of small arms is RIA-ES12. For description of the use of this cleaner, consult Ordnance Field Service Circular No. 161, December 5, 1940, and changes 1 and 2 to OFSC No. 161 dated March 1S, 1941, and April 5, 1941, respectively.

(2) In cleaning parts other than the barrel, wipe the receiver clean, care being taken to remove any dirt. from the pawls. Thoroughly clean all parts, using a. stick covered with a flannel patch if necessary. Then wipe all parts with an oily cloth so that a light film of oil is deposited on all surfaces. Aircraft instrument and machine-gun lubricating oil, U. S. A. Spec. No. 2-27, of the latest issue in effect should be used for lubricating all aircraft. machine guns regardless of ground temperature.

c. Preparation of guns for storage.-(1) Clean and prepare guns with particular care. All parts of the gun should be cleaned and wiped thoroughly dry with rags. In damp climates, particular care must be taken to see that the rags are dry. After drying, the bare hands should not touch the parts. These should be handled with an oily rag.

(2) Apply rust-preventive compound to all metal parts of the gun. Rust-preventive compound is a sluggish liquid which pours at a temperature of 81° F. It. is used for short time protection and should not be used on guns put. in permanent storage. It may be applied with a brush or by dipping. Application of the rust-preventive compound to the bore of the machine-gun barrel is done best by dipping a cleaning brush in the compound and running it through the bore two or three times. Prior to placing the weapon in storage, the bolt should be in the forward position with the firing pin released. It is good practice to relieve the firing pin tension whenever the weapon is not in use.

(3) The wooden supports in the packing box must be painted with rust-preventive compound before storing the gun. Place gun in wooden packing box, handling with rags.

f. Guns received from .storage-Sometimes machine guns received from storage are completely coated with rust-preventive compound. Using dry-cleaning solvent, remove all traces of this grease, particular care being taken that all recesses in which springs or plungers operate are cleaned thoroughly. All surfaces having been cleaned and dried should then be protected with a thin film of oil, applied with a rag.

NOTE.-Failure to clean the firing pin and driving spring, and recesses in the boIt in which they operate, may result in gun faiIure at normal temperatures and will certainly result in serious malfunctions if the guns operate in low temperature areas, as rust-preventive compound and other foreign matter will cause the lubricating oil to congeal or frost on the mechanism.

g. Dry-cleaning solvent.-This is a petroleum distillate of low inflammability and noncorrosive, used for removing grease. It is generally applied with a rag to large parts and as a bath to small parts. The surfaces must be thoroughly dried immediately after removal of the solvent. To avoid leaving finger marks, which are ordinarily acid and induce corrosion, gloves should be worn by persons handling parts after such cleaning. Dry-cleaning solvent will attack and discolor rubber.

h. Lubricating oil.-(1) Proper oiling is second in importance only to intelligent cleaning. It is of vital necessity for the working parts but should be used sparingly. Oil all bearing surfaces of the gun before firing, taking particular care to see that the exterior of the barrel is oiled at the breech end, also that the cover extractor spring, cover extractor cam, and cover detent pawl are oiled. Oil the cocking lever, groove in the bolt for belt feed lever, grooves in barrel extension to take bolt ribs, breech lock cam, switch, extractor cam, sear mechanism, and the guides of belt. feed slide.

(2) In oiling aircraft guns, special care should be taken not to use an excess of oil, since low temperatures that may be encountered will thicken the oil and may cause the gun to malfunction. The parts are best oiled by wiping with a well-oiled rag.

(3) Aircraft instrument and machine-gun lubricating oil should be used on all aircraft machine guns when operated in the air, regardless of the ground temperature.

(4) Aircraft instrument and machine-gun lubricating oil should not be considered as a suitable preservative. When so used it should be renewed at least every 48 hours.