Aircraft technical Basics: TM 1-409, Aircraft Armament and Pyrotechnics, 1941: II. - Ammunition

Section II - Ammunition

9. General.-a. Defective ammunition is responsible for a great deal of improper functioning of both aerial and ground-type machine guns. While malfunctioning of ground-type machine guns is usually easily remedied by the gunner, it is important that the aerial machine guns be relatively free from malfunctions, because the inaccessibility of fixed guns makes the clearing of jams impossible, or at least impractical. Many of the malfunctions can be prevented by proper inspection of ammunition before it is loaded into belts. Correct methods are necessary in the handling, loading, and care of ammunition from the time the ammunition is drawn until the links and empty cases move from the guns into the ejection chutes in order to prevent as many of the probable malfunctions as possible.

b. Cartridges with bullets having a diameter of one inch or less are generally classified as small arms ammunition. The caliber of the bullet refers to the diameter of the bore of the weapon in which it is to be used and is generally measured in inches, or as part of an inch. Caliber .30 ammunition is used in shoulder rifles, automatic rifles, machine rifles, and machine guns. Caliber .50 ammunition is used in machine guns.

(1) The components of cartridges in general consist of the case, primer, bullet, and propelling charge.

(2) The caliber .30 and caliber .50 cartridge cases have a conical-shaped body joined to the neck by a sharper cone called the "shoulder." The neck is the seat of the bullet and is very nearly cylindrical. The opening in the front end of the case is called the "mouth" and the rear end the "head." The circumferential groove near the head is called the "extracting groove" or the "extracting cannelure." The rim formed by the extracting cannelure is called the "extracting rim." In the head of the case is a depression made to receive the primer and called the "primer pocket"; connecting the primer pocket and the interior of the case is an opening called the "vent." Stamped on the head of each case are the initials of the manufacturer and the year of manufacture.

(3) The primer consists of a. cup, priming mixture, a disk of shellacked manila paper, and an anvil. The priming mixture is inserted in the cup and is held in place and protected by the paper disk. The anvil is inserted after the paper disk. A blow from the firing pin on the primer cup crushes the priming mixture between the cup and the anvil and causes ignition of the mixture. The holes or vents in the anvil allow the flame to pass through the vent into the case and to ignite the charge of smokeless powder.

(4) All caliber .30 and caliber .50 bullets are jacketed with hard metal. The bullet of the standard caliber .30 ball cartridge, M1, is constructed of a gliding metal envelope or jacket inclosing a lead slug or filler which is hardened with antimony. The bullet has an ogival nose and a tapered base, sometimes called a "boat-tail." About the middle of the bullet is a milled groove called the "cannelure." The cannelure allows the mouth of the case to be crimped into the bullet so the bullet will be held securely in the case, requiring a minimum pull of 75 pounds to extract it from the case. The gliding metal jacket is copper-colored.

(5) There are various types of powders authorized for use in small arms ammunition. The exact weight of the charge is not constant but varies with each particular lot of powder; the charge weight for each powder lot being adjusted to give the standard velocity within the pressure limits of the weapon in which it is to be fired. The powder used in the caliber .30 and caliber .50 ball, armor-piercing, and tracer ammunition is a coated, single-base, pyro powder of high nitration, granulated in the form of short., single-perforated, cylindrical grains. The grains are coated to retard the initial rate of burning so the pressure is better sustained throughout the barrel. The powder also contains a material which reduces the muzzle flash and aids in reducing metallic fouling.

10. Packing.-There are several methods used in packing small arms ammunition. Those methods most frequently encountered in the Air Corps are

a. In machine-gun belts, in various types of outside containers.

b. In cartons, in metal-lined wooden packing boxes.

c. In clips and cartons; in metal-lined wooden packing boxes (infrequently). The same packing boxes are used for both caliber .50 and caliber .30 ammunition with the proper markings on the box. The box for caliber .30 ammunition is designed to hold 1,500 rounds in cartons holding 20 cartridges. When used for caliber .50 ammunition, these boxes hold only 280 rounds packed in cartons containing 10 cartridges.

11. Lot numbers. Ammunition lot numbers are assigned at the time of manufacture to all types of service ammunition. The ammunition lot number is the means by which all service ammunition is identified and its importance is paramount to the armament officers or armorers. The incorrect reporting of an ammunition lot might result in the condemning of large quantities of serviceable ammunition, the issue of condemned ammunition, or the use of unauthorized ammunition which might result in serious accidents, e. g., the use of grade "R" of "MG" ammunition in synchronized aircraft machine guns would probably result in a shot propeller.

a. Characteristics.-Owing to the peculiar characteristics of each type of weapon, or the manner in which it is mounted or used, in the mass production of ammunition of a given type and caliber, the peculiarities and uses of these weapons must be considered. Therefore, the production orders and specifications call for the manufacture of lots for use in specific weapons.

(1) Production orders for ammunition for use in the service rifle require that the average of the net extraction effort shall not exceed 15 pounds. This is essential for uniform and reliable action in a manually operated weapon, but is of lesser importance in automatic and semiautomatic weapons such as aircraft and ground machine guns.

(2) Production orders for ammunition for use in synchronized and remote controlled aircraft machine guns require that the ammunition be of selected uniformity in dimensions and weight and minimum variations in rate of ignition. These requirements are essential to assure continuous firing during the combat use of aircraft guns where malfunctioning might result in destruction of propellers or create other hazards.

(3) Due to the rugged construction of ground-type machine guns, the control exercised by the operator, and lower rate of fire, less stringent test limits are required. As the amount of extraction pull is not a factor in machine-gun functioning, ammunition that meets the general specifications for accuracy, pressure, dimensions, etc., is satisfactory.

(4) Regardless of the weapon in which the ammunition is to be used, the requirements for velocity, pressure, and accuracy are uniform. However, due to uncontrollable factors, a machine-gun lot of ammunition might be less accurate or develop higher or lower pressures or velocities than a lot manufactured for ground machine guns or for rifles.

(5) From the above it is evident that the grade designations do not signify that one grade is better than another, except that it is better for some particular weapon. In time of peace, when ammunition is subjected to long storage, certain characteristics are revealed in surveillance tests which impair the efficiency of the ammunition for use in the guns for which initially graded. In this case the lots are regraded.

b. Unserviceable ammunition,-When small arms ammunition becomes unserviceable, it is designated as "grade 3" and withdrawn from service. Ammunition of this grade is unfit for service and must not be issued or used under any circumstances. Ammunition which cannot be identified because of obliterated markings or because of loose cartridges mixed in packing boxes will be considered as grade 3. However, unidentified ammunition will not be classed as unserviceable for this reason until every effort has been made, to establish its identity. Any grade 3 ammunition on hand in an armament shop will be immediately turned over to the Ordnance Department for disposition.

c. Summary.-To summarize, then, the grades of commonly used ammunition are

R-For use in rifles.

AC-For use in aircraft machine guns.

MG-For use in ground-type machine guns.

Grade 3-Not to be used in any weapon.

While every effort is made by the Ordnance Department to assure that ammunition drawn from them is of the grade authorized for use in various weapons, it should be a duty of each armament officer to refer to the Ordnance Field Service Bulletin No. 3-5 to ascertain whether or not that particular lot of ammunition is authorized for use in air-craft machine guns. The OFSB No. 3-5 contains lot numbers of all small arms ammunition in the service and the grade under which each lot number falls. The OFSB No. 3-5 is revised each year and any reference made to the OFSB No. 3-5 should be of course to the latest issue. All armament officers should thoroughly acquaint themselves with the OFSB No. 3-5, TM 9-1990 (now published as TR 1350-A), and AR 775-10. Small arms ammunition, as compared with other types, is not dangerous to handle. Care must be exercised, however, to keep the boxes from becoming broken and damaged. All broken boxes must be immediately repaired and careful attention should be given that all markings are transferred to the new parts of the box. The metal liner should be air-tested and sealed if equipment for this work is available.

12. Opening ammunition cases.-a. Boxes should always be opened by breaking the seals and unscrewing the wing nuts, and never by destructive force, since the wooden boxes are used again as long as they are serviceable. If the cover of the metal lining sticks, place a piece of wood inside the handle in order to get a better grip, and jerk loose with an angular pull. The metal lining is expendable but should be turned in for salvage.

b. It. is well never to open a case of ammunition unless it is known positively that the ammunition will be required for immediate use, since the lot numbers of loose ammunition so easily become lost, and since it is apt to deteriorate more quickly if taken from an airtight container.

c. The following are a few general rules in the use of ammunition:

(1) Ammunition must be protected against mud, sand, dirt, and water, and all foreign substances should be immediately wiped off.

(2) Ammunition will not be polished in an effort to improve the looks, but if verdigris or light corrosion forms, it should be wiped off with a dry cloth.

(3) The use of oil or grease on cartridges or in the chambers of high-pressure weapons is prohibited. The oil or grease will pick up grit, dust, and other abrasives which will be injurious to the mechanism of the weapon. The case of a dry or unoiled cartridge, when fired in a weapon with a dry chamber, expands under the pressure of the burning powder and adheres to the chamber of the weapon relieving the face of the bolt from a considerable amount of the pressure. If oil or grease is used, the case does not adhere to the chamber, but slips back and causes greater and possibly dangerous pressure to be placed on the face of the bolt.

(4) If a misfire occurs, the bolt should be. kept closed for at least 10 seconds, since it may be a hangfire. The term "hangfire" applies to a cartridge in which an appreciable delay occurs between the striking of the primer by the firing pin and the combustion of the powder charge. Should the operator open the bolt of the weapon immediately upon failure to fire, the cartridge might explode and injure him.

(5) All defective ammunition will be turned in to the Ordnance Department.

(6) Do not allow ammunition to be exposed to the direct rays of the sun for any great length of time. The heat generated will seriously affect the powder and consequently the muzzle velocity of the bullet, and will, in some. cases, cause dangerous pressures to be built up in the weapon.

(7) Whenever cartridges are taken from cartons, boxes, or bandoleers for loading into machine-gun belts, the belts will be tagged with the ammunition lot number so that in the event the ammunition is not fired it can later be identified and returned to its proper packing box. This tagging is necessary in order to prevent the loss into grade 3 of ammunition which could not otherwise be positively identified as to lot number.

13. Inspection and loading.-The cartridges of proper grade and the metallic belt links may both be drawn from the Ordnance Department. The ammunition boxes are opened with the necessary care, the cover of metal liner removed, and the cartridges removed from the cartons or bandoleers. When the cartridges are removed, they should be inspected for the following visual defects:

a. Season crack.-A split in the neck of the cartridge case caused by strain and improper annealing. Cartridges may be checked by pressing diagonally down on the bullet end. If defective, the season crack will be readily discernible.

b. Corrosion.-The discoloration and the building up of deposits on the case. In the minor stage, or plain discoloration, the ammunition is serviceable. When in the advanced stage, deposits build up on the case, the case has been weakened and is, therefore, unserviceable.

c. Mouth pull-down.-The mouth of the case was not concentric with the bullet when the bullet was seated, and the mouth of the case was pushed down, leaving a bulge which will prevent the seating of the cartridge in the chamber.

d. Shoulder bulge.-A pucker in the metal of the case at the junction of the shoulder and body. If too large, this bulge will increase the density of loading and cause the bullet to be unserviceable.

e. Oil dent.-A smooth surfaced dent in the body of the case caused by an excess of oil in the tapering die. The defect is negligible unless the dent is so large as to increase the density of loading.

f. Draw scratch.-A longitudinal scratch, varying in degree, along the case. This is caused by grit in the final draw die. If the scratch is very deep, the cartridge is defective and will open up on firing. If shallow, the round is serviceable.

g. Split-mouth.--A split in the edge of the cartridge case. Defective.

h. Folded neck.-Overlapping of the metal in the neck indicated by a longitudinal protuberance on the outside. Will prevent proper chambering of cartridge.

i. Scale.-Inclusions of impurities in the metal sometimes visible and sometimes hidden. If fired, the case will open up, and escape of gas to the rear will follow.

j. Indent and bur.-Dents and burs caused by rough handling. If too large, will cause an increase in the density of loading.

k. Grease.-This is similar to the fold.

7. Thick head.-The extracting rim thick. Improper functioning in the extractor.

m. Thin head.-The extracting rim thinner than normal. Failure to extract caused by extractor pulling through the thin metal.

n. Round head.-The head of the case rounded. This defect causes trouble in extraction.

o. Split bullet (tracer): Longitudinal cracks in the bullet. Caused by impure tracer mixture. This is a serious defect and should be immediately reported when found.

14. Hints on inspections.-a. Stand cartridges on heads to check for round heads, short, and long rounds.

b. Pass extracting rim through T-slot of machine-gun bolt to check for thin and thick heads.

e. Chamber suspected cartridges in barrel (removed from weapon) to check for proper chambering.

d. Inspection of metallic belt links may be perfunctory, merely checking for rust, corrosion, and broken or bent links.

e. When the cartridges are forced into the links by the loading machine, links that are defective will, as a general rule, immediately show up. As a further check, after the belts are loaded, the entire belt can be twisted slightly to check on the improper tempering of the links.

f. The last inspection given the ammunition is checking the correct alinement of the cartridges in each belt, care being taken to see that none of the links are twisted.

g. Before the ammunition is loaded into the airplane, the ammunition boxes and feed chutes should be checked for proper alinement with the gun, and the ammunition counter checked for freedom of movement to prevent the binding of the belt when moving from the ammunition box into the feedway of the gun. The ejection and link chutes should be cleared of any obstructions that could possibly cause a jam of the ejected cases and links which would cause a failure to fire.

15. Visual defects found after firing.-In the event that some provision has been made to retain the ejected links and cases, the following inspection given ammunition is a true indication whether or not any stoppages encountered during the mission were caused by the gun or were due to faulty ammunition.

a. Misfire (heavy blow)-shows that the ammunition was at fault and could have been caused by one of the following :

(1) Thick metal in primer cup.

(2) No primer pellet.

(3) No anvil.

(4) No vent.

(5) Various combinations of the above.

b. Misfire (light blow)-fault of gun, caused by

(1) Weak firing pin spring.

(2) Short or broken striker.

(3) Dirt or heavy grease in the bolt of the weapon.

c. Misfire (light blow)-fault of ammunition : primer seated too deep in primer pocket.

d. Hang fire-fault of ammunition.

(1) Small primer pellet.

(2) Damp primer pellet.

(3) Wet powder.

e. Pierced primer-fault of weapon : an imperfect striker.

f. Pierced primer-fault of ammunition: very thin metal in primer cup.

g. Primer leak (small or large)-fault of ammunition (leak of gas between primer and primer pocket).

(1) Minimum sized primer in maximum sized pocket.

(2) Normal primer in maximum sized pocket.

(3) Excessive pressure developed by the propellant.

h. Blown primer-fault of ammunition : primer blown completely from case.

(1) Excessive pressure developed by the propellant.

(2) A case with a soft head.

i. Primer set back (slight and heavy)-fault of ammunition: excessive chamber pressure.

j. Case leaked back-fault of ammunition : caused by draw scratch, season crack, scale, corrosion, or soft metal.

k. Case failed to extract-fault of weapon.

(1) Weak or broken extractor.

(2) Dirt in chamber.

l. Case failed to extract-fault of ammunition.

(1) Round head.

(2) Thin head.

(3) Soft head.

m. Split neck-fault of ammunition: neck splits during firing.

n. Split body-fault of ammunition: regular longitudinal split in body caused by draw scratch.

o. Stretch-fault of weapon: improper head.

p. Complete rupture-fault of weapon: improper headspace.

q. Partial rupture-fault of weapon : improper headspace.

r. Short round-fault of weapon: improper headspace.

16. Storage.-At the permanent establishment it is within the province of the Ordnance Department to maintain depots and magazines for the storage and care of ammunition. However, each squadron will normally have a small supply of ammunition and pyrotechnics which it will be the duty of the squadron armament officer to store. In the field also it will become the duty of each armament section to store and care for all ammunition issued to the squadron. The following are some of the general rules for storage of ammunition :

a. Storage of ammunition in garrison.-(1) Small arms ammunition should be stored and piled according to type and ammunition lot number and in accordance with any special requirements that may be covered in official bulletins. Extreme care must be used to prevent the mixing of ammunition lots in one pile. When small arms ammunition is received, issued, stacked, or restacked, reliable personnel should be in charge, and a check made of the ammunition lot number on each box.

(2) It should never become necessary in depots to store serviceable small arms ammunition in the open. Although there is a metal lining in the case, actual tests have shown that leaks in the liners are developed with handling and shipping, sometimes to an average of 8 percent in newly packed ammunition, while in the older lots leaks have been found in as high as 70 percent of the liners. The leaks, though small, would admit some moisture if the ammunition were exposed to the weather. Should it become necessary to leave small arms ammunition in the open due to unavoidable emergency, it should be raised on dunnage at least 6 inches from the ground and the pile covered with a double thickness of serviceable tarpaulin. Suitable trenches should be dug to prevent water flowing under the pile.

(3) Tracer and shotgun ammunition should always be stored under permanent cover. There are valid reasons for this. Tracer ammunition is subject to quick deterioration if allowed to become damp, and it is then apt to cause spontaneous ignition. Shotgun ammunition, except for oversea shipment, is not packed in waterproof, metal-lined boxes, and would therefore deteriorate quickly if exposed to severe weathering conditions. Tracer ammunition should be stored, if possible, separate from other ammunition.

(4) If fired into or placed in a fire, small arms ammunition does not explode violently. There are small individual explosions of each cartridge, the case flying in one direction and the bullet in another. In case of fire, it is advisable to keep everybody not. engaged in fighting the fire at least 200 yards from the fire and have them lie on the ground. It is unlikely that the bullets or cases will fly over 200 yards.

b. Field storage.-The general rules for storage in the field are not involved but more or less serve as a guide for the use. of persons charged with the handling of ammunition.

(1) Select a site that is free from all fire hazard and has ample drainage facilities; for example, a small hill or rise. It should be removed from personnel by at least 200 yards. In the event of war, a site would have to be selected which would be easy to camouflage.

(2) The ammunition boxes should be stacked at least 6 inches from the ground by use of dunnage (or beams) laid on the ground.

(3) Ammunition will be stacked according to type, grade, and lot number.

(4) Boxes will be separated by dunnage about 1 inch thick to provide ample air space.

(5) Aisles will be large enough so that ammunition boxes may be handled with ease.

(6) The entire stack should be covered with a double thickness of tarpaulin. The edges of the tarpaulin should be staked down and a drainage ditch dug around the edges of the tarpaulin.