Chris- You say resized cases give you some resistance?.How about ( if you can) Take a store round get the bullet out of it ( Kenetic Hammer) and Chamber that one. If it goes with no resistance ( as it should). You might be in line for a small base sizing die. I have had to get 2 of them already for stock rifles. Sometimes you just have a tight chamber, which is a good thing.
I would think the shooter/reloader saved new, factory, over the counter ammo from the beginning that would chamber and fire, I would think the same reloader would have save a few fired cases for reference. then I would think the shooter/reloader would use the fired case for reference when using a comparator when comparing sized cases with fired cases and with ammo he knew would chamber and fire.
Of course, all of that is out the window if the shooter/reloader is using a head space gage on a case. I use case gages, if I do not have one I make it.
F. Guffey