The MK 108 (German: Maschinenkanone—"machine cannon") is a 30 mm caliber autocannon manufactured in Germany during World War II by Rheinmetall‑Borsig for use in aircraft.

[citation needed] It was easy to maintain, and its compact size, low weight and electrical priming made it ideal for aircraft installation.

[citation needed] The cannon's distinctive heavy pounding sound and high rate of fire gave it the nickname "pneumatic hammer"[1] amongst Allied aircrews, who feared its destructive power.

[citation needed] In an Advanced Primer Ignition Blowback design such as the MK 108, the forward motion of the bolt is used instead of a locking mechanism to prevent this premature opening.

The bolt picks up a cartridge and chambers it, but instead of coming to rest against the breech face, it follows the case a small distance into the chamber—which is of course made specially longer in these guns to accommodate this manoeuvre.

[6] Once they emerge the weapon cycles like other automatic guns, with one significant exception: instead of simply ejecting the spent cartridge case, the 108 reinserts it into the empty link in the ammunition belt.

When the spring is fully compressed it begins to expand forwards again, reversing the motion of the bolt and recommencing the cycle.

[citation needed] The designers of the MK 108 opted for a high rate of fire and thus used a relatively light bolt, accepting reduced ballistic performance due to the corresponding necessity of a short barrel.

In this configuration, the cannons were mounted in the fuselage, aiming upwards and slightly forwards at an oblique (18 to 30 degree) angle, depending on fitment and aircraft.

This installation was so effective that discovery and news of its adoption was much slower than usual in reaching British night-bombing forces, as there were rarely any survivors from the attacks to report the new threat.

This system was fitted to some versions of the He 219 Uhu, late-model Bf 110 night fighters, Junkers Ju 88 & 388 and the Dornier Do 217N model.

In the latter case this produced a jet fighter with six MK108 cannons - with the fitment of the projected mass-produced, mid-VHF band FuG 218 radar.