The theory is that in some semi-auto rifles the base of the brass will expand so much that a conventional die will not resize it enough for reliable functioning. They are generally offered only for military calibers.
I have never owned any, don't need any for my M1 or Daewoo .223.
Small base dies were originally intended for resizing cases used in semi-auto firearms, to help improve feeding at fit. The small base-die is supposed to squeeze the case a little more than standard dies, near the base of the cartridge, hence its name. Some people believed that AR-15s were best fed with ammo that was resized with a small-base die, however the reality is that for regular chambers or for NATO or the hybrid chambers such as the Wylde chamber, the SB die was definitely not needed. In fact because these chambers could be so generous, an SB die would actually work the brass more than was desirable. If your AR-15 needed to have its brass resized in an SB die, you had an issue that needed to be addressed. So small base dies are not really needed, or are they?
Just like most things in life there are exceptions or special situations. In the pursuit of maximum precision in firearms, the trail goes through the chamber and it has been found that smaller, tighter chambers enable better accuracy out of firearms by placing the case in the exact same position every time. This tighter chamber must also be much more concentric to the bore of the barrel, but that's another story. If you have a precision rifle in which the chamber is cut to smallish proportions, you will want to use a small base die to resize your brass. Lots of people are under the impression that in such cases you would want to use a neck-sizing die, but some of us believe that the continual use of an SB die is actually the way to go for optimum accuracy in smallish, custom match chambers. For the folks who want to neck-size only and use a body die every few loadings, there are small base body dies available.
My two FTR match rifles have very tight chambers and I use SB FL dies for both of them. The brass life is not impacted by the use of these dies since they are always used and in fact you can barely feel any resistance during the resizing operation. The finished cartridges chamber very smoothly where cartridges assembled on regular dies will have issues. The practice of using SB dies for rifles with tight match chambers is spreading in competition circles and diemakers have recognized that and are offering small base dies in some of the calibers most often used in competition. The 2011 Redding catalog states:
“Now available in the following calibers for firearms with custom chamberings that are tighter than SAAMI specifications.”
It then lists a series of cartridges for which Redding offers small base dies either full length or body, or both. These calibers are .223 Remington, 260 Remington, 308 Winchester, 30-06 Springfield, 22 PPC, 6PPC, 6BR Remington, 6-284 Winchester, 6.5-284 Norma, 284 Winchester. They also offer their Type S full length bushing dies in small base for the 223, 308 6BR and 6PPC cartridges.
I have Redding Type S SB FL bushing dies in .223 and .308 calibers and have been using them to good effect for several years now.
If you have to ask the question, you do not need a small base die, but one day you may acquire a precision firearm with a custom match chamber that is very tight. At that time, you may need to consider a small base die.
Thank you for your responses. The first photo I have in my signature line below is a buck I bagged with a Remington 7400 in 30.06, I don't load yet for the 2 rifles I have in that cartridge but am thinking about and had saw refrence to using that type of die for a semi-auto. I think when the time comes I'll let the rifle tell me if it needs one.
I think that's a wise decision and it was a good question. When you decide to jump into handloading, I think you will find that a regular die will work just fine for the ammo that was fired in your rifle.
I've loaded for a BAR in .270 and recently, my Model 750 in .35 Whelen, both with FL dies, no problem. I like to use new or once fired brass in hunting loads, just as insurance. Years ago, my BIL's friend loaded for their Model 742's in .308; he had to use the SB dies, but they shot brass until it separated, well cracked, most of the time!
There is also a USB or ultra small base dies available.
The small base dies also have another function, they will bring a piece of range brass from an unknown chamber back to factory new size.
If you aquire used mil surp brass the sizer is a very good die to have, military chambers are very generous and on semi auto or full auto the chambers can leave a case very large at just above the webbing and expand the heavier areas also.
I loaded a lot of different calibers over the decades. I only found one rifle that needed SBDs. That was a friend's Remington pump in .30-06. The Lyman 06 dies, which I still use apparently would not size the case enough for some of the rounds. The pump did not have a lot af camming and loading strength compared to a bolt gun or a semi auto. We never did get the SBDs. We just test chambered all of the rounds to see what fed easy.