I have 3 1916 Spanish Mausers. All had excessive headspace when I got them (meaning, they'd close on a nogo gauge. I don't have a field gauge.). I've fired a few hundred rounds through the lot of them with no case head separations or any other failures. One shot well and is still in its original, military configuration. The other two shot maybe 20" groups at 100 yards.
The first of these, I got the Green Mountain 7x57, short chambered barrel from Midway, rented a reamer and gauges and everything went fine. It closes on the go, does not close on the nogo and has shot very well (Note that I'm chambering by hand. No power tools.).
The second, I got the same barrel, rented a reamer and gauges from the same company and am using the same procedure, but it has not gone well. When it closed on the go gauge, it took a lot of force on the bolt handle to chamber a factory-new round (any of many I tried), so I kept going, figuring the bottom line was that the ammo had to fit, and these guns all would close on a nogo gauge in their original configurations anyway... Well, now it closes on the nogo, but it still takes some force to chamber a round. Even so, when firing, I got 3 case head separations in 5 rounds, and it took considerable force to get the bolt back open for all of them. At that point I decided I was livin' dangerous continuing to fire the thing and set it aside.
So it seems like the headspace is excessive, but it's also overpressure? And note that this is the same type of receiver, with the same barrel, the same tools, same ammo, and the same procedure that worked before, so I'm baffled.
Any idea what could be wrong?