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. The cartridge has been primarily used by small game hunters and metallic silhouette shooters.
Although the .45 Winchester Magnum may be based on the .45 ACP and have the same Rim and Base dimensions, the .45 Winchester Magnum has no parent case. The .45 Winchester Magnum case is redrawn with thicker walls and longer case. The thicker wall dimensions of the .45 Winchester Magnum are designed to accommodate a higher internal pressure (40,000 cup) than that of the .45 ACP (18,000 cup [21,000psi or 140MPa]).