Not sure how you might do that by accident.
There's a vast difference in appearance between the 45 Colt and the 45 acp. The acp, being much shorter probably wouldn't cycle from the magazine, as the next round would extend from the mag tube, blocking the lifter from rising.
Assuming it was manually inserted into the chamber, it would not headspace. It could enter too far into the chamber and be unreachable by the firing pin. The 45 acp headspaces off the mouth of the rim, and there's nothing inside a 45 Colt (which headspaces off its rim) chamber to stop it from sliding in, all the way to the rifling. It would not fire in that position.
If the acp were to hang back far enough to be reached by the firing pin, it would probably fire almost normally. The base would be supported by the bolt, and the walls are near enough to the acp diameter that the case would probably not rupture, but it could expand enough to ruin the case. The other possibility is that it would make a "squib" and the bullet would get stuck several inches up the barrel. That latter possibility is what happens when you fire a .380 in a 9mm chamber.
Assuming yours was only an academic interest, this should cover the possibilities. Bottom line is, DON'T DO IT!!!!
Ha thanks. Wow you people are pretty harsh. Yes I guess I should have just looked at pictures online and assumed they were the correct thing because everything you see on the internet is true, if you‘re ignorant enough to believe just a picture you see instead of seeking advice from people who would have the knowledge you were seeking. I guess since I didn’t sit around and look at my dads dun magazines and actually went outside as a kid and interacted with others i shouldn’t own a gun. First i never tried to use any other ammunition then what was specified. Ammo is extremely scarce right now “that means insufficient for the demand” so with no ammo around to purchase i guess I could’ve just trusted a picture online that could have been put up by anyone of anything. But what a stupid question to just ask if my gun would be damaged... people who judge the way some of you have should never own a gun... that’s how bad decisions are made and wrong decisions. Having a simple one sided judgmental mind.Give the guy a break.
Mrawn88, let me WELCOME you to Marlin Owners!