In the past, when I have compared new to fired brass from factory belted chambers, the shoulders of the fired cases had been blown forward about .021. As long as this is a one time thing, I do not think that it causes a problem as long as proper attention is paid to properly setting the FL dies. One thing that I think is worth noting is that cases are not blown out to their largest shoulder to head dimension in one, or even two firings, so when I am working with once fired brass, I set the FL die to give the same head to shoulder dimension as the fired case. Of course all fired case measurements are made with the primer removed. Once you have a case that is tight at the shoulder, you can use that dimension to bump back from.
Some time back, I ordered a reamer for a friend's .300 Weatherby project. He wanted a tight neck, and freebore that would allow 180 grain hunting bullets to touch the rifling at a loaded length that would allow functioning through the magazine. Having learned some things from previous projects, I sepcified the chamber body diameters the same as factory. We also ordered a FL die reamer. The die set that he had for his factory barrel included a one piece neck die, that we used for our pressure test loading. The shoulder locations of the chambering reamer was specified so that it would only be blown forward .006 on the first firing, instead of .021 that was the case with the factory barrel. The reason for the pressure testing was the short throat, since we had no data. We started relatively low, and worked up to well over loading manual maximum, working carefully, and measuring belt expansion as we went. The sizing was neck only, and even though we tested well above what we picked as a maximum load, when we were finished, the single case that we had use for the test, loading at the range as we went, easily chambered without sizing. I attribute this to the factory clearances in the body of the chamber. The first time that the new barrel was blooded, a northern New Mexico elk was shot high in the shoulder at 600 yards , and dumped on the spot. I would say that the project was a success. I forgot to mention it earlier. The chamber and turned neck dimensions were such that the one piece neck die's neck ID gave perfect neck tension, and perfectly straight sized brass.
