Mauser headspace question

[Elkins45]

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 gauge. 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 gauge bottoms out. Shouldn't a too long gauge like the FIELD protrude enough so that there actually is some excessive headspace 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 gauge (or a Go gauge) protrude from the barrel? What is the 'correct' specification for case head protrusion?

TIA for any help/advice.

[Buzzard Bait]

Me thinks you need a gun smith. One option might be rechamber to longer cartridge like 6mm remington.
bb

[Fleet]

In my limited experience, the correct fix here is to set the barrel back one thread, and then go back in with the chambering reamer until the headspace is correct. I don't know of proper dimension for "protrusion". If there were such an animal, you wouldn't see all the chat, video's, books etc. talking about doing it the other way.

In fact, with the headspace off that badly, I'd question whether the other barrel dimensions are right. Do the barrel shoulder and the breech face both make contact with the abutment in the action and the front face of the action?

[edward5759]

Gauges only give you a range.
make yourself an actual head space gauge and then use a Wilson case gauge it is designed to indicate if the case is to long or to short, measured from the base to the datum Line. you can then find out if someone did a chamber in a Ackley or some other. If the did neck size only the fired cases. Before SAMMI was formed they would fit the reloading die to the rifle, not the rifle to SAMMI because SAMMI wasn't there.
Kinda lost art of reloading. A chamber cast will also tell you what you a dealing with. I have seen all kinds of 6mm chambers cut on Mauser's. 6mm-284, 6mm-06 244 H&H magnum, find out for sure what you have first. Drop me a pm if you want.

[Scorch]

I have an Erfurt 1920 stamped Mauser action that has been sporterized and fitted with a prechambered heavy barrel in 243. . .

The problem with it is that the bolt closes on a FIELD gauge

If the bolt closes on a NO-GO gauge, you unscrew the barrel and inspect the locking recesses in the action for peening. If they are peened back, face the locking recesses, and send the action off for re-heat-treating; if not set the barrel back and recut the chamber so that the bolt will close on a GO gauge but not a NO-GO gauge. Pretty simple, really.

Before SAMMI was formed they would fit the reloading die to the rifle, not the rifle to SAMMI because SAMMI wasn't there.

From SAAMI's website

The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) is an association of the nation's leading manufacturers of firearms, ammunition and components. SAAMI was founded in 1926 at the request of the federal government and tasked with:

•Creating and publishing industry standards for safety, interchangeability, reliability and quality
•Coordinating technical data
•Promoting safe and responsible firearms use

So it is extremely unlikely that the rifle in question was chambered before SAAMI was formed, since the 243 Winchester was introduced in 1955.

One option might be rechamber to longer cartridge like 6mm remington

243 Winchester is larger than a 6mm Remington in every dimension between the head and the neck, so you cannot simply rechamber, the reamer will not cut a clean chamber.

Me thinks you need a gun smith

Bingo! Instead of letting internet experts guess at what the problem may be, let a smith look at it. My guess would be that it may be OK or need to be set back, but without having it in hand, it is just a guess.

[Jimro]

Tools needed: Dial calipers, go guage, lathe. It is an easy job if you have access to a lathe. If not, pay a gunsmith.

Remove the barrel from the action. An M98 action has an internal "C Ring" in the action where the open part of the C is where the extractor fits. You need to have a dial caliper to make some measurements.

First, measure how far out of the chamber your GO gauge protrudes. Use the protruding bottom of the caliper to get this measurement.

Next, measure the distance from the C Ring to the bolt face. Same method.

Ideally the numbers for gauge protrusion and C Ring to bolt face are the same. In your case the number for Bolt Face to C Ring will be significantly larger.

So what you need to do is calculate the difference, which is simple subtraction, of course you want to leave thousandths or two for "crush" so you don't undersize your chamber.

Now there are two torque surfaces on a Mauser barrel, the part against the C Ring, and the shoulder against the reciever ring. There are those that like to put all the torque on one surface or the other, I like to torque against both equally as I think it is the most secure fit.

And there you have it, two cuts on the lathe to recut the shoulder and chamber face and you torque to spec on the reciever and you have set the correct headspace.

Jimro