Unless you've got an old revolver chambered in 45 Colt, lead bullets for the 45 ACP and 45 Colt are sized .452, and jacketed bullets are sized .451. Barrel dimensions vary on old Colt SAA, etc., sometimes up to .454. If you're loading bullets without a cannelure into 45 Colt cases, be careful about bullets setting back in the cases under recoil. I've never loaded bullets designed for the 45 ACP into Colt cases, but I've used thousands of .452 255 SWC's designed for the Colt cartridge in 45 ACP cases to use in bowling pin matches. Those big 255 grain SWC's function flawlessly thru my Glock 21 and S&W 4506, and knock pins off the table with authority. OAL is very important, so check it carefully.