I've been reloading for 10 year now. But I just started doing 375H&H. I loaded some virgin brass up (300gr hornady, 77gr H4350) and it shot fine (just ran the necks through the FL die). I tried resizing my fired cases with a brand new 375FL sizer from Lee and it is shaving brass from the case just above (about 0.1") the belt even with plenty of lube applied. I ran some unfired cases (all cases are Remington)through the same die and everything seems normal. I measured the unfired cases above the belt and they are slightly undersized compared with the standard case drawing. The fired cases directly above the belt are within spec as well but as you go farther up the case they grow in diameter slighly (0.007") over unfired cases. Do I have a really slopy chamber? Sizing die is undersized? I can get some pics up this evening if it would help.

maybe a burr on the new die? check the edge where it meets the case.
how much does the belt expand compared to new never fired brass?
Dave

Had the same trouble with a Lee .458 WM.
(Well, they are cheap)
I just rubbed the bottom inside edge of the die with a bit of emery paper. Didn't take much to fix it.

Called poor quality control...take some emory paper and knock the sharp edge of the entrance. That is another reason I don't use Lee, prefering RCBS.

I think you guys are right about the sharp edge. This is my first time with lee dies and I was pleasantly surprised until I sized my first case. Emory paper = what? 1000grit sand paper?