I have a Ruger BHK in the .45 Colt/.45 ACP. If you look down the .45 Colt cylinder, from the rear you will see that the chambers are not cylinders as in other calibers (.44 Magnum)....the first 1/2" is a conical taper which then becomes a cylinder. If you insert a factory round in the cylinder, there is considerable play side to side because of the conical area. When you resize the cases with a carbide sizer, the sizer sizes them into a cylinder. However using a RCBS steel sizer die, the die will size the case to better for the cylinder, as it too has a sight taper. So I resize .45 Colt brass w/ the steel sizer die...I must lube the cases; I then clean them using SS pins & dawn detergent (also cleans teh primer pockets). The sized cases fit teh chambers better and I believe that case life is better.