The Rheinmetall-Borsig Maschinenkanone 108 (MK 108) 30mm automatic cannon first appeared in 1940, created by Rheinmetall-Borsig as a company endeavor. In 1942, the Reichsluftfahrtministerium (RLM; Reich Aviation Ministry) issued a requirement for a heavy cannon for aircraft which could engage Allied bombers and cripple or even destroy a bomber with minimal ammunition usage. Rheinmetall-Borsig dusted off the MK 108 and presented it to the RLM who, satisfied the cannon met the requirements, accepted the MK 108 for acquisition with the cannon entering operational service in mid-1943. The first aircraft to be equipped with it was the Messerschmitt Bf 110G-2 heavy fighter and the Bf 109G-6/U4.
The MK 108, by itself, had a weight of 128 pounds with a barrel length of just under 2 feet (23 inches). The action utilized the principle of Advanced Primer Ignition (API) blowback. As is sometimes said of German engineering, this action required exceptional and precision construction and assembly to ensure smooth operation. API blowback works by igniting the primer within the round while the bolt is still moving forward and before the round is fully chambered. The rearward force of the detonating primer both slows the bolt's forward motion to prevent it from slamming into the breech while leaving enough momentum to send the bolt rearward to repeat the cycle. Such an action allowed the MK 108 to have a lighter bolt amd have a low recoil.
The MK 108 had a cyclic rate of fire of 650 rounds per minute with a muzzle velocity of 1,640 feet per second. This was rather low in comparison to other aircraft cannons and created a dangerously short effective range of 200 to 300 meters to ensure hits. While the MK 108 had a far longer maximum range, the round suffered from rapid drop which made accuracy at range poor. Testing showed that at 1,000 meters the round drop was as much as 135 feet from the aim point. However, the MK 108 was never designed to engage fighters (though, a single hit to a fighter could cause significant damage) but the much larger target profiles of bombers.
The caliber of the MK 108 was 30x90RB. RB stands for Rebated which means the rim of the round has a smaller diameter than the base of the round. The main benefit of a rebated round is that it can be easily converted into differing ammunition without having to change the weapon's action mechanism. While the MK 108 had several ammunition types, the most common used in combat was the 30X90RB Minengeschoß (Mine-Shell) round. A thin-walled, steel cased round, it packed 85 grams of PETN (Pentaerythritol Tetraniotrate) explosive into the nose. Allied bomber crews were able to easily hear the MK 108's distinctive sound when it fired, nicknaming the weapon the “pneumatic hammer.”
While the MK 108 had some success when equipping propeller driven aircraft, when the MK 108 was installed in turbojet aircraft, the short effective range made scoring hits a very dangerous affair for pilots. The Messerschmitt Me 262 jet fighters' speed was much higher than Allied bombers and so a successful engagement against bombers from the front or rear (an attack profile to minimize the number of defensive machine-guns that can take the aircraft under fire) was very difficult. With only 360 rounds to feed four MK 108 cannons, Me 262 pilots eventually developed an attack profile in which they flew above the bomber stream and once a target was selected, the pilot would put the Me 262 into a shallow dive which used the plane's speed to avoid escort fighters. Then, the pilot would continue the dive behind and below the bomber then pull up, bleeding off speed. This put the Me 262 around 1,000 meters from the rear of the bomber with the pilot opening fire at 500 meters then pulling away to avoid a collision at 200 meters. The rocket fighter, the Messerschmitt Me 163, had an even higher speed and pilots attacking bombers had between 2-3 seconds to execute an attack before having to pull away.
The lack of range saw the Me 262 equipped with 55mm R4M unguided rockets on underwing racks, 12 per wing. On approach, and outside of the bomber's heavy machine-gun range, the pilot would unleash the rockets which would create a spread pattern. Just one or two hits from a R4M was enough to down a bomber.
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