I have an Erfurt 1920 stamped Mauser action that has been sporterized and fitted with a prechambered heavy barrel in 243. How I ended up with is a long and uninteresting story.
The problem with it is that the bolt closes on a FIELD gage. I made myself an action wrench and turned off the barrel. After a bit of research and measuring I have come to the conclusion that the chamber was originally cut too deep. I say that because the extractor contacts the breech face before the FIELD gage bottoms out. Shouldn't a too long gage like the FIELD protrude enough so that there actually is some excessive head space when the parts are held together outside the action? That's my big question.
I'm thinking I need to shorten the chamber by facing off the barrel a bit. My other question is how much should a FIELD gage (or a Go gage) protrude from the barrel? What is the 'correct' specification for case head protrusion?
TIA for any help/advice.
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These things do not lock me up, you say you have a 1920 Mauser that has been sporterised to 243 (something? you do not know what) so you used a field (reject) gage and discovered the bolt will/would close and you do not know by how much, then! you removed the barrel.
NOW! before you fall for some more bad advise like, when you correct the head space you will need a go-gage and a no go-gage to go with the other gage you do not/did not need, 'back to the stuff that does not lock me up' I check head space with a go-gage, no go-gage and the beyond gage, to put it another way I check chamber length, in thousands, with a field (field) reject gage, that includes go-gage length and no go-gage length and all the little length between, and I make head space gages that are -.012 shorter in length that a minimum length/full length sized case for short chambers and gages that are .024 thousands longer than a go-gage length chamber, that is 36 different length gages.
Back to removing the barrel, if the barrel is removed or measured before installing head space can be determined before the barrel is screwed in to the receiver, on occasions I miss the dimensions by .001 thousands.
What would I do? Where would I start? The Mauser barrel seats/faces on a 'C'/torque ring 'THEN' seats on the front receiver ring against the shoulder at the end of the barrel threaded shank. Both distances can be measured, but, the measurements are not easy to keep up so remember, the bolt face must seat first against the 'C' ring in the receiver, as to what I do to make sure, I screw the barrel into the receiver until it contacts the 'C' ring, then use a feeler gage to determine the gap between the front of the receiver ring and shoulder at the end of the shank on the barrel, .0025 is a nice number, a smith that thinks he is going to crush metal when torquing the barrel to the receiver is silly, at bets the smith is going to remove the slack between the threads and marr metal. Point being, you did not check to make sure the barrel contacted the 'C' ring before the shoulder on the barrel contacted the receiver ring.
You have a field reject gage, If I had a filed reject gage and I had the barrel off, I would drop the gage into the chamber THEN measure the gage protrusion (case head protrusion), in the perfect world with a go-gage the case head protrusion on a Mauser is .110 + or - very few, with a no-go gage the protrusion would be .114 and with a field gage the case head protrusion would be .119, back to a pre-chambered barrel, the chamber could be correct but the shank could be short meaning the short shank could be holding the barrel off of the 'C' ring.
Again, I am the one that determines the length of the chamber first from the face of the bolt to the shoulder of the chamber. Again, I have a M1917 Eddystone that has .016 thousands head space, again, this stuff does not lock me up, I form cases that have an additional .014 thousands length added between the head of the case and shoulder of the case.
And, NO! I am not a fire former, I chamber a round, pull the trigger and eject once fired cases. When I fire form? the weak pass out and the strong get dizzy.
F. Guffey
