Mauser rifles have a buttress thread where the gas shield fits into the back end of the bolt. I've got a special tool bit stashed away ground to turn that thread when I need to fabricate a copper heat sink to protect the threads when I'm welding on a custom bolt handle. The thrust bearing face of the thread is angled at 5 degrees, and the other side is 45 degrees. The thrust of lifting the bolt handle and compressing the firing pin spring is carried by the flatter angle. There's no really good reason for it, as other, less sophisticated rifles use a conventional 60 degree V thread on the bolt parts, but Paul Mauser over-engineered everything about his designs- - - - -way back in the late 1800's!
Jerry
