How much would it effect my ammo to use the SB die?
I have small base dies, I also have BAR dies. I have dies that are mistakes, if it was necessary to reduce the case diameter in front of the case head/above the shell holder I can do that. I do not find it necessary but 'just in case' I have the dies. And I have feeler gages, when sizing a case for a short chamber I use the feeler gage. When I want to know if it is necessary to use a small base die I start with a feeler gage.
Then there is measuring the diameter of the case above the case head. By design I find it impossible to size the case head, the shell holder with a deck height of .125" prevents the die from sizing the case below .125" above the case head + the radius of the die.
F. Guffey