Canadian Polsten 20-mm towed quadruple-barrelled AA Gun, Swords & Poughshares Museum, Kars, Ontario. The Polsten 20-mm Quad AA Gun was a low cost Polish development of the 20-mm Oerlikon gun. Both the Oerlikon and the Polsten used similar 60 round drum magazines; however, the Polsten could use a simpler box magazine with 30 rounds. It remained in service into the 1950s.