Originally Posted by DWalt

There wasn't actually a .45 ACP round as we now know it at that time, but there was a dimensionally very similar round for the M1905 Colt autopistol having a 200 grain bullet. And at that time that round had not been adopted for government use either. The very first .45 rounds for use in an autoloading pistol were hand-made modifications of .45 Colt cases. No, NOT .45 Long Colt cases, as there never was a .45 Short Colt, just a .45 S&W, which was sort of a shortened .45 Colt originally intended for the Schofield but used in both it and the Colt SAA. As pointed out, the M1909 cartridge was essentially the .45 Colt cartridge with a larger diameter rim for use with the Colt New Service revolver, the last handgun officially adopted by the US Army prior to the adoption of the Model 1911. It didn't work so well in the Colt SAA due to rim interference between the chambers unless you were happy with a 3-shot SAA that went BANG-click-BANG-click-BANG. The ammunition used in the triple-lock during these tests was probably the .45 S&W, as that is what the Army standard .45 round was in 1907 (despite its having adopted the .38 Long Colt cartridge and a Colt revolver much earlier, in 1892).