The 3 cm MK 303 Flak and twin-mounted 3 cm MK 303 Flakzwilling (M44) were experimental 30 mm anti-aircraft guns developed in Nazi Germany.

They fired the powerful 30x210mm round and only 222 were produced.

[1] The gun was to be installed on Type XXI submarines as AA defense[3] and its use as AA defense replacing 2 cm Flak and 3.7 cm Flak weapons was also considered.

303 Flakzwilling on Flakpanzer IV "Kugelblitz" was considered, but rejected.

In the 50s, the MK 303 was produced in Czechoslovakia under the designation M53, with the clip feed replaced by a 10-round box magazine.