Hypothetical question: If I was to have a re-chamber job done on a custom bolt gun and at the same time have the shoulder cut just enough to turn the barrel about 1/8 turn to bury the old caliber stamp, would this change really make that much of a gap at the wood/barrel surface? Any one ever done this? I'm inclined to think there wouldn't be much of a gap. FWIW, I like to free float my barrels anyway. I am assuming the barrel has right hand threads as well. Thanks. CB

Unless your barrel has a sharp shoulder like a M70 Featherweight or a rear sight boss you would never notice the difference.

If the barrel thread is 16 threads per inch the barrel would move to the rear only .007" of an inch. The change in diameter would be much less. Probably less than .001.

Thanks. I thought so. I should have mentioned that the barrel would be one without sights.

I did exactly that so I could chamber my 30-06 to a 308Norma

Not to get too mathy...take the reciprocal of 16...1/16 which equals 0.0625" and is called the "thread pitch".

The reciprocal of ANY thread per inch or tpi is the thread pitch. 1/20 is .050", 1/12 is 0.0833", 1/14 is 0.0714", 1/18 is 0.0555"...those tpi's cover just about all the common rifles...Rem, Win, Ruger, Mauser, Weatherby, etc.

Divide that 0.0625" by 360°, one complete rotation, and that equals 0.0001736"...each degree you turn the barrel it moves 0.0001736".

Divide 360° by 8 which is 45° times 0.0001736" and that comes out 0.007812".

0.007" is about the thickness of two sheets of printer paper., and the gap it leaves is what SR4759 says...1/3 the thickness of A SINGLE SHEET OF PRINTER PAPER.

In other works about the amount that a barrel OD increases or decreases whenever it is shot a few times and cools off.

Basically not a heckofalot.

Some gunsmithing reference books like Brownell's "Gunsmith Kinks" covers this aspect of gunsmithing and some don't...I think it is something that should be covered thoroughly, But...and get right down to it, it's just basic geometry once you think about it.

Now, I'm not saying it made me piddle down my leg a little the first time I had to do this "thing"...because I'm a hairy legged, bare knuckled, arse kicking he-person...that was just coffee I spilled...I've done it many times since when I get to mucking about with some wildcat/factory comparison re-chambering
...some things never cease to amaze me. LOL

Works for me. I learned something new today, actually a lot. Thanks for the numbers. Quite a simple feat to calculate once I had the TPI number. CB

Whooeee...I can't believe I sad that part about the piddling. Can't drink, never could and see what comes through the alcoholic haze.

The nervous part did happen to me back WAY back in the day when I was still in high school, but that was because my gunsmith mentor, shop teacher had his chin on my shoulder, breathing down my neck, watching every move.

I didn't mess up too much and that bit of haw-haw served as a better teacher in itself...misteakes often do.

One thing I didn't mention...the amount of calculated setback needs to be added(subtracted rather) to the tendon shoulder, the barrel face and the chamber, and the same amount of movement occurs to the barrel taper...so the barrel taper moves back the same amount. You, of course, knew that.

You get a 0.007" gap where the straight section and the beginning barrel taper occurs. Still 0.007" is easily filled with a thin coat of bedding epoxy. You have to get into your differential calculus to come up with the rate of change in gap with respect to the rate of change in rearward movement in the circumference of the barrel. Not really needed un less you are practicing your calculus. Hahahahaha