50 caliber machine gun
There are two types of mounts to hold machine guns - one is 'Post Mounted' for manually depressing the firing trigger - see top illustration and the other 'turret or wing mounted' where the trigger is operated by an electrical solenoid because of the remote location to the operator - see photo right of silver boxes with wires on side of black housings. LEFT is a weapons bay of a P-51 Mustang.
Source: Wright-Patt AFB
There are several hundred small parts to a .50 cal machine gun. They are assembled into two main functioning sections.
One is the exterior housing that includes the barrel holder and the rear main housing complete with belt-feed cover - and rear butt plate with handles and trigger mechanism.
The second group of parts are the internal working parts of the .50 cal gun which are removable and made-up of the barrel and bolt assembly.
Source: Wright-Patt AFB
The task of caring for these machine-guns fell to a ground crew known as the armorers. The armorers came to the aircraft after a combat mission, and removed the guns, carrying them to a shop where they were cleaned and inspected and made ready for the next combat mission.
The .50 cal machine gun is an air-cooled weapon capable of firing 550 rounds of ammo per minute. The mechanism operated from the recoil of a cartridge being fired. The firing of the cartridge set all internal parts in motion.
The 50 caliber machine gun was somewhat accurate for a distance of 1,000 yards, just over a half-mile. The effective engagement ranges is under 400 yards, with the best effective range being under 250 yards. The weapon would place a bullet pattern of about three feet at a distance of 1,000 yards, if locked in a rigid vise. However, the vibration of the gun firing caused a slight movement in the gun mounts and the actual bullet pattern was assumed to be about thirty feet in diameter. Of course the 50 caliber projectile would travel a distance of more than a mile but the pattern was much larger due to vibration of the mounts.
Though the 50 caliber machine gun fired at a rate of 550 rounds per minute some gunners on bombers, particularly waist gunners firing single post mounted weapon, found a way to increase their fire power. The 'phenolic' disks in the back plate were of the same size as the American quarter, a twenty-five cent piece, which became valuable in a machine gun. These fiber disks were replaced with American quarters. It required about ten dollars worth of quarters to replace the phenolic disks in the back plate of each gun. The hardness of the coins caused the recoil of the bolt to react quicker, thus increasing the machine gun firing rate from 550 rounds to 650 rounds per minute. A gunner altering the gun in this manner had to be very careful because he could overheat his gun. But the increase in fire power was worth it when enemy fighters made a pass at the bomber formation that lasted only mere seconds. Over time the silver quarters would compress from the beating of the rearward force of the steel bolt and the gunner constantly checked the quarters for size and replace them when necessary. Replacement averaged about every ten combat missions.
From top to bottom .303 British, .5 inch Vickers, .50 Browning, 20mm Hispano cannon
Source: Wright-Patt AFB
Convergence is the range where all the bullets come together at a pre-set range. Warbirds kills are scored mostly by destroying certain select areas of a target plane as quickly as possible. Ideally, the thickest part of the fuselage should be targeted; normally the portion containing the pilot, engine and other vital components. To localize fire on one particular area is usually much more effective than spreading the bullets all over the target. Hitting the target plane at the set convergence will allow the pattern of bullets to be at their densest and will be the most destructive. Convergence is an important factor in all planes which carry their guns in the wings. For planes carrying their guns in the nose or cowling convergence is not an issue. This is particularly true in a fighter such as the P-38 Lightning.
For planes that carry their guns in the wings, convergence plays a serious factor. This is especially true for the .50-caliber machine-gun armed planes which generally must rely on a burst of converged fire to destroy an enemy plane.
Getting inside convergence has its own set of problems. The rounds are packing a massive kinetic wallop, but the projectile pattern on the target is not an ideal one. Because the heavy machine guns do not have the explosive effect of cannon they rely almost exclusively on their kinetic energy to inflict damage. For this reason getting in tight is, as always, the best policy. A 50 caliber round at 50 yards packs about twice the punch as the same round at 400 yards.
Deflection shots are difficult but can be effective at ranges at or near convergence. The pilot who has only one type of gun, for example the six 50's of the P-51 Mustang, doesn't have to worry about having different types of rounds with different ballistic characteristics all going down range at the same time. E.G. mixed cannon and machine gun rounds - the rounds will have different rates of drop, time of flight, and energy loss etc.) This can make deflection shooting a bit easier, but it's only really effective near convergence. Snapshots are possible, but usually will not impart enough hits to inflict a catastrophic kill on the target. The most common scenario in a snapshot is a "lucky" hit on some vital component, ideally the pilot. Since a large bank of these types of weapons throw a lot of rounds down range, the chances for such a "lucky" hit are somewhat better in planes with large banks of machine guns as opposed to those with just a few cannon.
When a P-51 Mustang's six 50 caliber machine guns fire on a target at proper convergence, it has the same kinetic punch as the head-on collision of two automobiles. In as little as one or two bursts, a properly targeted aircraft can be destroyed. A well trained pilot does not waste ammunition. When a P-51's outboard four machine guns are empty, the plane held a reserve of only 260 rounds - 130 for each inbound weapon.
The P-51, armed with six .50 caliber machine guns - three in each wing- held 400 rounds for each inboard gun and 270 rounds for the other four thus a total of 1880 rounds.