Does the process of improving a standard factory cartridge always require the barrel to be set back?

I've "passed" on some gun deals, because I was sure the barrel wasn't set back. One was a pre-'64 M70, the other a FN Mauser.

Am I making a mistake??

packrat

If it is a rimless cartridge and is done correctly, the answer is yes. It is possible to fireform brass to fit a chamber which is too deep but it won't work as the AI cartridges were intended to. That is, it won't allow the use of factory ammunition in the improved chamber. Regards, Bill.

Bill is absolutely right. Here are a couple of problems that will arise if the barrel is not set back: (1) If the AI reamer is run to the correct headspace, it will not "clean up" the 18.5 degree STD shoulder and you will end up with a "dual" shoulder in the chamber and (2) If the AI reamer is run deeper to remove the 18.5 deg shoulder, you will have excessive headspace. I'm going to post a pic that will hopefully illustrate this point.

I think the image will show now. I read a post further down and someone said you can clean out a chamber w/o setting the barrel back. If the headspace datum is partway down the shoulder (i.e. just about all '06 based cases) this is not possible IMO.



[ 06-17-2003, 08:56: Message edited by: Lars G ]

I reamed the chamber on my 257 RAI with a 257 Roberts case as a headpace guage. That was a mistake, as the firing pin will push the brass another .004" forward [because the shoulder only touches at the line of the rear of the neck]. When I fire form now, I have to have the case lubed, so it will slide back to the bolt face and make the shoulder FL in my chamber. If I don't lube, the cases measure .004" shorter at the shoulder.

This is my only Ackley chamber and the most accurate of my ~50 center fire rifles. I don't know if the Ackley aspect causes the accuracy, there are so many other varibles.

Theoretically, a standard chamber should and would "clean up" with an A.I. reamer without setting the barrel back.

The problem is that virtually all factory chambers are somewhat oversized (because the factories don't want you to fail to easily chamber ammunition that is also a bit oversized, and because SAAMI maximum specs are very generous to allow the factories to start with grossly oversized reamers so that they get more chambers out of them before they wear down to SAAMI minimums). Because the A.I reamer has to be run deep enough to clean up the oversized chamber, the resulting chamber may be rather long without barrel setback, especially for factory ammunition which is notoriously UNDERSIZED (for the same reasons factory chambers are OVERSIZED.)

Not having the slightest interest in firing factory ammunition in any of my guns, whether standard or non-standard caliber, it would make me no difference if an A.I. I purchased had "excessive" headspace when shooting factory rounds. Besides, with the claw extractor on either the W70 or M98 that you mentioned, firing too-short factory loads would still be safe, if not pretty.