from the old forum, dunno who posted the question, but the subject is a good one:
anybody know what mould would work best for a revolver with a 451 barrel and 456 throats?
Select or make a bullet with a diameter to match your cylinder throats. Or if the nose profile permits, you can use one .001 oversize to the throats.
I have the same problem in a Ruger. .4555 Cylinder throats. The most accurate bullet is one that drops from the mold at .456. Lubed and unsized they work well. No leading.
Proper bullet fit will maintain a good seal and keep the high pressure gases behind the bullet where they belong. If not, gases will bypass the bullet, eroding the circumference of the bullet and it's base by gas cutting. This material is then sprayed ahead of the bullet and deposited. When the bullet goes by it will them "iron" it into that area. Throats, barrel, etc.
The next question that arises is "What about pressures?"
Two thing to consider is: Are your chambers large enough to permit case expansion and the release of the bullet? Plug gauge your chambers next to the throats and the dimension should be .002+ larger that your loaded rounds with the proper size bullet for your throats. The worst situation you could have is an undersize chamber that will not allow the proper size bullet to be shot by not allowing the loaded round to chamber at all.
Next, If you are trying a bullet that is plus size, +.001, insure that the +size bullet doesn't rest on or is forced into the cylinder throats. Permit a little leade and let the +size bullet get a little jump going into the throats. .010 is enough. When you do this there is neglegable differences in pressures.
Don't worry about the barrel size of .451, when the bullet gets there the bright light from behind will insure it's fit, keeping a tight seal. The forcing cone and barrel are the sizing die.