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Hi, Yes, you can fire a . 45 GAP cartridge in a pistol chambered to use the . 45 ACP, but there is no good reason to do so, and the performance is likely to be unreliable. I won't do it.

It may still function but normally 45 ACP headspaces off of the case mouth and GAP is too short so it would be held in place by the extractor or fall loosely into the chamber. Either way reliability/safety is less than firing 45 ACP so there is little reason to try it in my mind.

I wouldn't do it until I contacted Springfield and found out from them first. The difference in size could result in getting a live round stuck in your firearm. Springfield customer service is pretty good about answering your questions quickly.

Jon, this is not recommended due to the amount of bullet jump into the lands and the corresponding pressure spike when the bullet does hit the rifling whereas the 45ACP's bullet is already seated at the rifling when firing commences. Semi-Auto's have a tuned/timed lockup for the specific chambered cartridge to properly cycle the action unlike a revolver say .357/.38Special that just sits in a cylinder.

I wouldn't do it until I contacted Springfield and found out from them first. The difference in size could result in getting a live round stuck in your firearm. Springfield customer service is pretty good about answering your questions quickly.