Regarding military chamberings, they could indeed be overlong.

What happened in my 303 SMLE chamber was that the shoulder was more than 1/16" further from the bolt face than nominal. The case shoulder wouldn't roll forward, so the case body stretched just in front of the web, and swelled out by quite few thou because of the tapered chamber. I could FL resize the cases, but if I did it just squashed back the swelling in front of the web and disguised the problem whilst also probably aggravating it by working the brass. I got several case head separations on reloads - none of them dangerous - so I eventually gave up and stuck to factory rounds.

It might well not be possible for this to happen with a less tapered and rimless round like .308, but I think the advice above to measure diameters around the case head and just ahead of the web is good.