The .45 Winchester Magnum is a .45 caliber rimless cartridge intended for use in semi-automatic pistols. The cartridge is externally a lengthened .45 ACP with a thicker web to withstand higher operating pressures. The 45 Win Mag is nearly identical in dimensions and loading to the .45 NAACO developed by the North American Arms Corporation for their Brigadier pistol, developed to supply to the Canadian Army after World War II. The army ultimately did not adopt the pistol and its non-NATO standard ammunition.