22, 2011 The 45 ACP The  .45 ACP  (11.43x23mm) ( Automatic Colt Pistol ), also known as the  .45 Auto  by  C.I.P. , is a  cartridge designed by  John Browning  in 1904, for use in his prototype  Colt   semi-automatic  .45 pistol and eventually the M1911 pistol  adopted by the  United