nearly same thing here
Bought a 640kd used, put about 7 rounds through it and something seemed "off". Got to checking and noticed a couple of the brass were split/cracked down the side, all of the backs of the cases(closest to boltface) were bulged out a bit. Also found that after bolt was closed, with a round in the chamber, I could still push the bolt handle forward .029 in. Going to mill a set of go-nogo gauges and check this further. Just on quick inspection bolt appears to be ok, but the boss seems to maybe show some signs of wear. Gauges will tell the tale, will try to update after findings.