I have several .45 Colt SS NMBHs with .45 ACP cylinders. All my .45 Colts required reaming the chamber throats. They are about 10 yrs. old and had .450 or so throats. I reamed them to .4525. (Newer ones may not have this problem.) I sent them to Ruger to get .45 ACP cylinders fitted. I had to ream several of the .45 ACP cylinders as well due to them being too tight. I also had the .45 ACP cylinders modified to shoot .45 Auto Rim ammo as well. Mine are very accurate with both the .45 Colt and .45 ACP/AR cylinders.