I cut threads and chamber for a 6mmRem 1903 Turkish Mauser #3 taper Douglas 12" twist barrel.
Then i realized that the 257 Roberts/ 7x57mm go gauge was too long.
I should have used a 6mm Remington go gauge.... which was in an unopened package. I assumed they were the same, but I must have known better at one time, I would not have ordered the 6mmRem go gauge.
The SAAMI chamber drawing for 257 Roberts shows the 0.375" shoulder datum at 1.7937".
The SAAMI chamber drawing for 6mm Remington shows the 0.375" shoulder datum at 1.7767".
Sounds like 0.0170" too much headspace..
So I put the barrel back in the lathe, dialed in the annular disc of the shoulder edge... I cut 0.0170" off the breech in 3 places.
but wait... there's more..
The SAAMI chamber drawing for 257 Roberts shows the 0.375" shoulder 20 degrees 39 minutes.
The SAAMI chamber drawing for 6mm Remington shows the 0.375" shoulder 26 degrees.
That doubles the error.
So I cut again.. but I cut too much.
So I got out the reamer.
But I reamed too much.
So I cut a little bit off the shoulder and measured. Then I cut some more. Then I measured, then I cut.
Same thing for the breech face.
Then I have to chamfer so the step of 0.012" from bullet to case mouth will feed.
So I made two big mistakes:
1) Wrong go gauge
2) Thinking I could cut an accurate amount without going in baby steps.