Recommended length of machine gun length for B-29 gunners. Source: Combat Crew Manual, XX Bomber Command, December 1944.
LENGTH OF BURSTS
There are several factors to consider in arriving at an answer to the question of how long a burst it is practical to fire. The ammunition has a high degree of accuracy. At 600 yards, when fired from an accuracy rifle held in a V-block, it will group in a circle 18″ in diameter. When fired single shot, using an aircraft machine gun on a tripod mount, tests have shown a 20″ circle of fire. In a burst of 10 or 12 on the same mount the group was approximately five feet. When longer bursts were fired, it was observed that the gun soon lost accuracy, even though it remained relatively stationary in the mount. When over fifty rounds were fired, in one burst, the projectiles tumbled in flight and dispersed over a 75 foot area at 600 yards. When the barrel has been overheated, it will be found that it cannot be relied upon for further accuracy even though the lands and grooves measure up well and the barrel, to all appearances, seems good. If the exterior of the barrel has a burned appearance, it should be tested by ordnance before further use. When a barrel becomes over-heated it expands to such an extent that the muzzle velocity decreases several hundred feet per second. This decrease continues as the barrel continues to expand, until a point is reached where tumbling of the projectiles takes place and controlled fire is reduced to a few hundred feet. The accuracy of the fire delivered, therefore, depends not only on how steadily the gun is held, but also on the length of the burst, and the condition of the barrel. If a gunner fires short bursts of three to five rounds, constantly using his sights, he will have a tight group and a high degree of accuracy. This is the most effective method of firing your machine guns.
