What’s the difference between a .45 ACP and .45 GAP?
“GAP” is new to me.
.45 GAP is a special cartridge that was introduced by Glock. Incompatible with .45 ACP.
“GAP” is new to me.
It's a custom round specifically made and promoted by Glock. It's a shorter .45 cartridge.
Kind of a flop like 10mm.
Modern powders have improved to the point that less powder is required for the same velocity and bullet weight achieved in the original .45ACP design, resulting in lots of empty space in the cartridge. Glock created the .45GAP cartridge a couple of years ago to utilize modern powders and shorten the brass casing to eliminate this empty space. The end result is a cartridge that matches the performance specs of the .45ACP but is ~1/4-1/3 shorter in overall length. This permitted Glock to redesign their G21 and G30 .45ACP handguns and reduce the grip dimension. The new models are (largest capacity to lowest): G37, G38, and G39 in .45GAP.
Primary drawback is ammo availability.