Bird Dog II,
Mike called it. My PM for .308 reads about 0.002" off on the high side, so your's could do it on the low side just as easily. Get a good quality .308 GO gauge from Dave Manson or Clymer or Pacific or JGS, and use that to get the zero (I've got a set of inexpensive Forsters that Midway sells, but they are no more accurate than the PM, being off by as much as 0.002", so I recommend the more expensive brands). Zero should be equal to that minimum chamber length that the GO gauge represents, so measuring the GO gauge will give you an offset to add or subtract from your readings to correct them. SAAMI's spec for new brass allows for up to 0.004" interference fit, + 0.000/-0.007", so 0.003" under the GO gauge's length would be the lower limit (the interference fit is possible due to the chamber being wider than the cartridge, so the cartridge can swell, but I know of no brand of brass made that long).
Your reference to "base sized" makes it sounds like you may be thinking standard full length sizing dies and small base full length sizing are the same. They're not the same size. The small base style is smaller than the standard style, usually both in shoulder length and base diameter (hence the name). So let's go through the differences and why they exist:
In general, the problem with brass is elasticity. It springs back some from each stretching event, within short limits it does so in proportion to how much its size is being changed, but it never springs back completely. For example, you have to fire a neck-sized case a number of times before it really starts to get snug in the barrel, so new brass fired just once in your chamber is not as close to chamber size as four times neck-sized and fired brass is. I usually figure headspace taken off a new case fired once in your gun is around 0.001" smaller than the actual chamber headspace.
Different brands of brass have different degrees of work hardening imposed during forming and therefore are not equally springy. They can also have different degrees of shoulder and neck stress relieving (neck "annealing"). Therefore they can be resized with greater or lesser ease. This presents a problem for the FL sizing die designer. What size does the die need to be to return the case to its original size? Or do you even want to return it all the way to original size? Might it not be better just to make it enough smaller to feed into the same chamber from a magazine?
Different designers seem to have different ideas about those issues, so different die brands have somewhat different sizes. Board member Humpy says he's got dozens of dies ground to slightly different lengths so he can adapt to different brass and different chambers more precisely. That's more trouble and expense than most of us are willing to go to, but it illustrates the principle of what's required for exact results. Fortunately, most guns don't require that.
In general, any standard FL sizing die will size a case small enough to feed into the chamber it was originally fired in. That is, it will reduce the length at least a couple of thousandths, which is usually considered good enough for magazine feeding, and the diameter at least one thousandth. In reality, unless your chamber is very tight, a standard die will usually resize it more than that if your chamber dimensions are generous.
If you start with brass that was fired in a chamber larger than your own, the standard die often won't squeeze it hard enough. This is common with once-fired military brass. It was fired in a NATO spec chamber. These are both a little longer and wider than commercial chambers. As a result, the spring-back of the once-fired military brass coming out of a standard FL sizing die is not uncommonly too great for a case feed into a SAAMI spec chamber.
In those instances, you buy the small base sizing die. These are typically a couple thousandths shorter and a thousandth or two narrower than a standard sizing die. That is normally enough additional sizing to get the big brass down to commercial chamber size. Once that has been done and the brass is reloaded and fired in your chamber, then a standard sizing die will usually size it enough thereafter, because sizing is no longer starting out quite so big.
Once in awhile you find a gun that requires extra narrow cases to feed properly or that extracts so hard it stretches the brass in addition to chamber fireforming. This is usually just an issue in self-loaders. The brass for these particular guns can require the use of the small base die every time.
