If you take it down, remove the barrel and extractor, how much do the resized cases stick out of the chamber? How much clearance is there between the barrel face and the firing pin bushing with empty chamber and the action closed? (with or without extractor). If the cases stick out equal to or more than the clearance, then it's the cases or the recess for the belt at the rear of the chamber. If the cartridge belt is hitting the chamber cut out for it (like the headspace-on-the-belt guys say) and the case is sticking out too far, then no amount of screwing down the resizing die will help. Then you are faced with needing barrel alterations or firing pin bushing alterations - or maybe a new lot of brass. Sometimes tolerances stack up in our favor, sometimes not. Sounds like yours are stacked toward the tight side.
You might try smoking that brass for it's whole length with a candle, and closing the action with standard force as far as it will go. That could show if you're hitting on the belt or shoulder. I'm hoping when you say it won't close you are working just with the brass and not loaded rounds
I'm pretty sure I read that in your posts. Let us know what you find out.
Tom
