http://www.shootingtimes.com/ammunition/new_45/ "The internal case wall profile is significantly different from .45 Auto cases to permit seating .45-caliber bullets without bulging. The most obvious difference is that the .45 GAP uses a Small Pistol primer to put distance between the primer and breech/action elements present in 9mm-frame pistols. It is unsafe to cut off .45 Auto cases to make .45 GAP cases. The most significant issue is the difference in primer size. A Large Pistol primer that fits the .45 Auto case will over-ignite safe powder charges developed in proper GAP cases with small primers, and high pressure will result. The .45 GAP case has a much different internal taper, so cutoff .45 Auto cases will bulge when bullets are seated to recommended lengths." Be safe out there. Since it's not run of the mill brass, I hear many talking about cutting down 45ACP cases. Doesn't look like a good idea. Not to mention that 45ACP cases are not really high pressure ones.