Mk 21 naval gun

The Mk 21 3-inch/50-caliber dual-purpose gun was widely used both during World War II and in the postwar period as the primary gun armament for ships up to destroyer escort/frigate (DE) size and as the secondary armament for larger warships.

There were five major 3-inch gun mount variants, all of which used the Mk 21 or Mk 22 3-inch gun.

The Mk 22 mount was widely used in World War II-era ships, but now is found only in former U.S. frigates and small auxiliaries transferred to...