Re: RCBS SB Dies???
I believe SB stands for small base, not short base.
The SB dies are designed to resize the brass casings to a smaller diameter in order to allow more reliable feeding into the chambers of the semi-auto actions or pump actions, where the additional resistance of a tighter fitting cartridge into the chamber could result in failed feeding.
The reason they would not be recommended in a bolt action rifle is that the added down sizing of the brass is not required in bolt action rifles, and the greater the brass is worked down during resizing and then expanded during firing, the lesser the brass life. You'll get fewer reloads because of the extra work hardening of the brass cases with the SB dies.
A lot of guys get away with the normal resizing dies even with the AR15s. I'm using the standard RCBS dies in my AR15 and have not experienced any jammed actions so far. Haven't shot it alot yet, but so far, so good. If I start experiencing feeding malfunctions, I'll buy a Redding SB body die and reduce my reloaded .223 ammo so it can feed into the chamber with greater ease (a looser fit in the chamber).