Hispano MKII machine-cannon
Wright-Patterson AFB museum
Machine or auto-cannon differ from normal machine-guns in that they fire explosive rounds generally of greater caliber. The most common type in the American Army Air Force was the 20 mm Hispano MKII cannon. The characteristics of these weapons could vary considerably, and their lethality generally improved as the War wore on. There were numerous models of cannon of the 20 mm type manufactured by all the combatants. Other caliber's including 15 mm and 30 mm cannon were also used by the Axis forces. This class of weapons strength was exceptional lethality, at the expense of lower ammunition supplies and a slightly lower rate of fire. However, most machine cannon had a rate of fire about just a little more than half that of heavy machine guns. The 50. caliber Browning discharges 550 roun ds per minute while the 20mm fires about 325 rounds per minute. A two-second touch on the trigger of a Browning discharges about nine rounds while a 20mm dispenses 5 rounds or less.
Machine-cannons are exceptionally lethal, but they do have their limitations. Most American fighters with 50 caliber guns deployed six to eight guns apiece. Most cannon-armed planes deploy one or two cannon, and in a few cases four cannon. While the aircraft has tremendous hitting power, it is not going to have as many rounds going down range as aircraft armed with machine-guns. It can do more damage, but there will be a lower probability of getting hits. The pilot will have to be an exceptional shootist.
Deflection shooting is generally more difficult since cannon effective range is limited by a larger bullet drop, and energy loss. Energy loss (i.e. drop in speed) has a lower impact on the lethality of an explosive round than a machine gun round, but a cannon round that doesn't hit the target is useless regardless how powerful it is. The practical range for most machine cannon is about 400 yards. Ideally ranges should be much closer. A convergence setting of about 175-250 yards is recommended. Ideally the cannon-armed plane needs to attack with as low a deflection angle as possible. Deflection shooting is particularly difficult with these heavy rounds. Of course, if hits are acquired there will be some serious damage.
Often cannon-armed aircraft carried a secondary armament consisting of some type of machine gun. This means that the pilot has to deal with weapons that have different characteristics. For instance, a Supermarine Spitfire that fires at a certain deflection angle versus a target might have ideal deflection to hit with his machine guns, but the cannon rounds might all miss because the ballistics of the cannon rounds are different than those of the machine gun rounds. In practical terms this means that the cannon-armed plane has to fire at shorter range and lower deflection to gain maximum effectiveness. This also applies to long range fire. Trying to effectively get two different types of guns on target at long range can impose an almost insurmountable task to the pilot. For this reason the pilot should probably only choose to discharge one type of weapon for that 'spray and pray' shot at longer ranges. The exception to this rule was the nose-mounted armament of the Lockheed P-38 Lightning.