I'm reading Jerry Kuhnhausen's Ruger shop manual. He talks about an optimum 11 degree forcing cone angle to keep "cone related bullet deformation" at a minimum. I don't understand this terminology, what is this deformation? Thanks
If the forcing cone is at a bad angle it can cause the bullet to be reshaped and this can cause accuracy problems. The correct angle and length of the forcing cone eases the bullets transition from chamber to barrel.
Also have to have the correct cylinder to barrel gap and alignment of cylinder to the barrel.
Checkmate, Revolver forcing cones are located just in front of the cylinder face at the very beginning of the barrel. The forcing cone's job is to "force" the cylinder to move slightly so the cylinder's throat will align with the bore. It does this when a round is fired and the bullet nose contacts the forcing cone. The cone literally funnels the bullet into the bore.
By the time the bullet nose strikes the forcing cone, velocity has built up considerably. This can cause the bullet to be distorted, shaved, or damaged to a point where it is no longer perfectly round. Once the bullet exits the muzzle, a distorted out of round bullet will whiffle its way down range. The more the bullet is distorted, the worse accuracy will be.
Most factory forcing cones are shallow cut with a 5 - 8 degree reamer. Additionally, it's very common to see machine marks, galls, and divots in a forcing cone ... even in brand new guns. Chamfering the forcing cone with an 11 degree reamer does two things. First, it removes any corruption and second, it provides a better transition from the barrel's mouth to the lands. This results in less bullet damage and better accuracy. It also looses some velocity.
Do all revolvers need this? No! If your gun groups good and doesn't build up fouling in the cone, you don't need to waste your money. On the other hand, if you have problems with accuracy related to your cylinder not lining up with the bore perfectly, or the cone has obvious signs of corruption, your gun will indeed profit from getting the forcing cone chamfered. The reason for 11 degrees is because it is slightly wider than the factory cut and is known to work well. You would never want to chamfer with a smaller angle because a compound cut will result that will demo your bullets.