What is the exact purpose of SB dies in reference to a .223 chamber used for autoloaders?

If your refering to small base dies, they keep the size of the brass to the minimum dem which is actually slightly smaller than F/L sizers so to ensure that they work through the action of a semi or pump.

The SB dies focuses on the brass area from the shoulder to the case head where I've heard it sizes an extra .002" or so more than the FL.



We use the SB for the initial sizing of brass fired in other chambers. For subsequent loadings use either FL or neck sizing from then on.

Say what?