I am of the opinion that small base sizing is safety critical for Garand mechanisms as firearms built around a Garand type action slamfire both in battery and out of battery. Having an interference fit case and a sensitive primer is the most common characteristic and highest risk situation in out of battery slamfires in Garand mechanisms.
However, the firing pin is totally contained in the bolt of a AR action and therefore given a tight case, about the worst thing that will happen is a failure to chamber or a failure to extract.
AR10 actions have a heavy bolt carrier and this action will crunch fit slightly tight cases to the chamber. But, even so, I would recommend small base dies as they size cases to factory dimensions than standard sizing dies.
I have 308 and 30-6 gages, cut by Gene Barnett which are a little out of the ordinary. He cut these gages with his chambering reamer. Standard cartridge headspace gages are cut with a special reamer that is wide in the middle. A standard cartridge headspace gage measures length, not “fatness”. A reamer cut headspace gage will show you if the case is too long and too “fat” for that chamber.
I have a number of 308 small base dies, and I still have my Lee standard die.
I sized a number of my match cases in the Lee die. All of them dropped in the reamer cut case gage. So, if you said you don’t need small base dies, you would be correct most of the time.
So now I had to scratch around trying to find cases that would prove my point.
These two cases are once fired range pickup brass that I found in my brass box. I had to go through about 20 cases before I found a set of really ballooned cases. On the right is the Barnett reamer cut gage.
If you notice one case has completed dropped into the Wilson cartridge headspace gage, while the other has not dropped into the Barnett reamer cut gage. This shows how much they have swelled up after being fired. Must have been a big chamber.
The second picture is of the fattest of the group after sizing in with Lee Die. I trimmed the thing to make sure that the case neck did not interfere with the throat in the gage.
Hopefully you can see that the case did not drop all the way in the case gage. At least on its own. It would have taken a good hard push to get that base all the way in.
This is after resizing in my RCBS small base die. I could not find the RCBS box, so the case/gage are sitting on a Redding small base box. However, that little additional sizing removed the interference fit.
Sometimes cases are so ballooned that even a small base die won’t reduce the case to factory dimensions. It all depends on the chamber the round was fired in.
I know it is extra effort to size cases with small base dies, if you use a good lube like Imperial Sizing wax or RCBS case lube, the effort is somewhat reduced. Still for all the extra bother involved in sizing with small base dies I'll do it for my Garands and M1a's. With those rifles I don’t want any resistance to chambering, I don’t want any delay to bolt closure. Because as the bolt stops and the lugs are turning, that darn free floating firing pin is just tapping the heck out of the primer.
