Originally Posted By: Rhound
Someone explain this small base die thing to me? I load for my AR using standard carbide lee dies and functions perfect.
Standard base dies FL sizing dies do not return the case walls to factory "new case" dimensions.
Most ARs, Mini-14s, and other gas gun chambers are very over-sized, to aid in functioning - some bolt guns are also over-sized. When you fire the round, the base of the case, just above the head (called the "web"), expands enough that you can see an obvious bulge.
If you hold a straight edge against the bulged side of the case, you will see that it does not lie perfectly flat against the edge, there is a large space between the straight edge, and the head of the case.
If you hold a straight edge against the bulged side of the case, AFTER you have sized it with a standard sizing die, you will see that there is still a bulge, though it is now smaller - the straight edge STILL does not lie perfectly flat against the edge, the sizing is incomplete. In most guns, it will chamber.
The amount of this bulge will vary from gun to gun, depending on the size of the chamber, which is dependent on the size of the reamer, and how many times the reamer has been sharpened.
Brand new reamers will make the largest chambers, and the cases with the largest bulges... an old reamer that has been sharpened many times, or a match reamer, will make the smallest chamber, so it's fired cases will have the least bulge, or almost no bulge, because their chamber walls are the smallest in diameter.
The older a reamer is, and the more times it has been sharpened, the smaller the chamber will be... until the reamer is too small to use anymore.
Sizing dies are the same, since they are cut with reamers too - a sizing cut with a new reamer will not size the walls of the case completely down to the original dimensions, but that case will fit 80% to 90% of the chambers.
It won't size cases enough to fit chambers that are tight - like match chambers or chambers that were cut with an old reamer.
Now, if you have a small or match chamber, and you want to use surplus cases, or range pick-ups, you may have problems with a standard sizing die, cuz it might not size the case enough for your rifle, and you will get wedge jams, where the case slides into the chamber, and the rear end of it, jams in the smaller chamber, and it won't completely close, and it won't open either - it is stuck part way into the chamber (ask me how I know this
) Pounding a wedged loaded round out of an AR or Mini is not a lot of fun, no matter how you do it.
The Small Base die is specially made to the smallest size possible, so it will size ANY fired case to the original wall diameter, so if you hold a straight edge against the sized case, there will be no light showing between the straight edge, or only 1 or 2 thou.
In summery - if you have an AR, Mini, or other gas gun, with a large or sloppy chamber, you can get by with ANY FL sizing die....
... but if you have an AR or other gas gun, and the chamber was cut with a small (old reamer) or match chamber, you will eventually run into problems when your case won't size down the minimum case diameter... especially if you use range pick-up, or mil-surp brass.
When guys ask about sticky case problems on this site, and someone suggests that they try a small base die...
... there are always a few guys that reply, "Well, I'm using a standard die, and don't have any problems, so you don't need a small base die... "
Well, sure, it goes back to - if you have a loose chamber you will do fine with a standard FL die... but if you have a tight or minimum chamber, you WILL need a small base die.
RCBS had so many complaints about FL dies not sizing AR cases enough that they make special AR die sets - the only difference in the AR die sets is that they contain a small base (SB) FL sizing die.
Some guys think that the SB die will "over work" the brass - that is pure nonsense. The brass at the web, just above the case head, is thick and tough - it is one of the strongest parts of the case... and when sized, it doesn't move that much - I have NEVER seen a case fail from being over sized at the web.
I hope that throws some light on the "Small base die" thing.
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