In order for the .45 ACP cartridge to work in the old Colt and S&W revolvers, the half moon clips were devised. These clips required the cylinder to be shortened on the hind end, which allowed part of the head of the .45 ACP case to be exposed. The .45 Auto Rim had to have a thicker rim to make up the difference of the clip, and place the case head in the same relative position as the ACP case with clips.Thanks guys for the info. I never even thought about a difference in rim thickness between the 45 ACP RIM and the old 45LC. I wonder why there is a difference in rim thickness.