What is a sizing die marked SB for and why? Afriend told me I needed to use them in my slide gun ammo or I will have trouble. I have not had any yet. Was he just dealing with a particular gun that had a sloppy oversize chamber?
SB means small base and they size the base down more than regular dies, for UNDERSIZED chambers. I'd guess that you don't need them if you aren't having any trouble, but watch for problems with brass that has been fired multi times and new batches of brass. Brass springs back some after firing, but it will be a little less each time. A new batch might be a bit fatter than your old brass.
A bolt action can cam out brass that will stick other actions, so small base dies are often recommended for pumps, levers, semi-autos & singles (break & falling block).
Chief, to add to that reply, if you only use brass that has been fired in your gun, you will likely never have problems (as you've seen so far).
If you got some military brass or such laying on the ground at the range, chances are that it might have been fired in an oversized chamber and there is the possibility that standard dies would not resize it sufficiently to get it back where it would fit in your chamber.
I'd say your friend is honestly trying to help, but clearly doesn't understand the situation!
I'll second Mike's comments. Small base dies are mostly used these days when commercial brass is used in military-type firearms like a Springfield Armory M-1A, etc. This is particularly true when using .308 cases in some 7.62X51mm NATO-chambered arms. Contrary to popular belief, the two are not identical cartridges. Very close but not identical. In some rifles small base dies really help functioning.
All of the above is true. I haven't really anything to add except for an interesting little bit of information I thought I'd pass on to you guys. One of the local gunsmiths has been turning out several 22 Cheetahs (for anyone not familiar it's simply a 22-308 Win. basically), and everyone was having problems with "what they thought was overpressure", I looked at the fired cases and noticed that they all looked like small scrape marks in the web area of the cases. This was on three different rifles. I suggested that they try my SB 308 sizer and then running them through the 22 Cheetah FL. Bingo, no more problem. They were all just a tab undersized in the rearward section of the chamber. No more problems. Just had to add this. No real bearing on the original question, I know.
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