on ' s ▁own ▁K AA ▁and ▁K AB ▁weapons ▁in ▁the ▁process . ▁A ▁US - built ▁model , ▁the ▁M 1 3 9 , ▁saw ▁some ▁use ▁on ▁vehicles . ▁ ▁Development ▁▁ ▁Develop ed ▁in ▁the ▁post - World ▁War ▁II ▁era , ▁the ▁H S . 8 2 0 ▁fired ▁a ▁ 2 0 × 1 3 9   mm ▁round ▁developed ▁from ▁the ▁Swiss ▁ 2 0 × 1 3 9 ▁FM K ▁( and ▁FK ▁ 3 8 ) ▁as ▁a ▁replacement ▁for ▁their ▁earlier ▁ 2 0 × 1 1 0   mm ▁design ▁of ▁the ▁widely ▁used ▁H S . 4 0 4 . ▁The ▁new ▁round ▁was ▁one ▁of ▁two ▁post - W W II ▁European ▁ 2 0   mm ▁designs , ▁the ▁other ▁being ▁the ▁ 2 0 × 1 2 8 ▁developed ▁by ▁O er lik on ▁for ▁a ▁similar ▁role . ▁The ▁H S . 8 2 0 ' s ▁high ▁ 1 1 0 0   m / s ▁m uzz le ▁velocity ▁and ▁equally ▁high ▁ 1 0 0 0 ▁round ▁per ▁minute ▁firing ▁rate ▁made ▁it ▁a ▁form id able ▁weapon . ▁Nevertheless , ▁the ▁rapid ▁introduction ▁of ▁even ▁higher ▁performance ▁revol ver ▁can n ons ▁meant ▁the ▁H S . 8 2 0 ▁was ▁never ▁as ▁popular ▁in ▁aircraft ▁role ▁as ▁the ▁H S . 4 0 4 ▁had ▁been . ▁ ▁The ▁weapon ▁was ▁more ▁widely ▁used ▁in ▁a ▁variety ▁of ▁mount s ▁for ▁the