The 45 GAP is not simply a shortened 45ACP. The brass is thicker to operate at higher pressures to achieve 45 ACP ballistics in a 9mm/40 size gun as stated above. The rims are very close. I often get a GAP case in with my 45's when reloading and they fit the 45 shellplate. Matching the 45acp ballistics was not a problem due to the number of excellent propellents avaialble today. The 45acp is quite capable of going beyond it's century old ballistics with todays propellents. Those of us with 45acp revolvers can shoot GAP's with moon clips if one wanted to. I was reloading some 455 Webley's a while back and noticed that the GAP is basically a rimless 455. Their case lengths are almost identical. I guess what goes around comes around. Bob