Renegade19sc,
It's normal for cases to spring back a thousandth or so after being inflated by firing pressure to fill the chamber. That springback is why your RCBS PM is reading some of the cases short. Only if you are loading right up on the edge of getting sticky bolt lift, will a case mirror actual chamber size accurately. This is why you can get away with neck sizing cases only: until they have been fired a number of times, and assuming the load is not near maximum, that springback sees to it they still fit back into the chamber easily (though eventually they expand enough that it ceases to be so).
As far as headspace goes, keep in mind that the NO GO gauge is merely an upper limit for brand new chambers. It's not an upper limit for a rifle that has been fired. That would be the purpose of the FIELD REJECT gauge, which, for .308 is typically another 0.004" longer than the NO GO gauge. (IIRC, the SAAMI standard actually allows for 0.006" beyond NO GO, but the commercial gauges I have don't go that far.) So, if your rifle started out right at the NO GO length and had, as most commercial rifles do, at least slightly imperfect bolt/receiver lug contact, plus tool marks on both when new, then it might not take much shooting for that chamber length to to settle in another thousandth longer than when the factory measured it. On the other hand, if you had a new custom rifle with lugs that had been lapped to create even contact over a wide lug area before the chamber had been cut to fit it, depending how beefy the action was and how hot the loads were for the chambering, it could shoot thousands of rounds without appreciable change.
So, what does any of this matter? Unless you are trying to use the same batches of reloads in a number of different .308 rifles, not a whole lot. The fact is, as Mr. Guffey has described many times, you simply gauge your resizing to match the chamber. The most common way to do this is via a transfer measurement you take from cases fired in that same chamber. There are many ways to go about getting a comparative measurement for sizing a case to fit a particular chamber, but since you already own the RCBS PM, you should use that. It's a very good tool for this purpose. You are simply looking to adjust your full length sizing die to resize the case with its shoulder set back an additional thousandth or two from its as-fired size.
That procedure is, after all, what many wildcat cartridges have been. A chamber is made bigger than the parent cartridge's chamber (outside SAAMI standards for the parent). The cases are blown out by firing to fit that extra space, then subsequently resized to match that changed dimension. It works fine. Thus, even a chamber that goes over FIELD REJECT length can be treated as a very minor form of wildcat simply by sizing its cases for that chamber instead of for a SAAMI standard compliant chamber. All you lose is ammunition interchangeability with guns that have shorter .308 Winchester chambers.
