Barrel set back guidelines

[Codywy]

I have a few decent quality Remington heavy contour take off barrels that I picked up. I have been machining parts for gun projects over the years but have never attempted a chamber job. I wanted to re-chamber one of the 223 chambered barrels to 222 as I already have a rifle in this, dies etc. but it has a skinny tube on it. I have read Hinnant's book more than a few times as well. It looks like I will have to cut off the threaded barrel tenon and start fresh. This will of course move the chamber farther up the barrel and more into the first part of the barrel taper. The question I have is there some type of chamber to barrel OD ratio you should not exceed? I bought these barrels cheap as I wanted to use them for more of a learning process then expecting any improvements in accuracy. If I screw a few up thats OK its better than attempting your first chamber job on a Kreiger! Thanks

 

[Eddie Harren]

If you do some careful measuring, you should be able to "save" as much shank OD as possible. You could then "pick up" the thread that is left.

 

[Codywy]

Yes I checked that out and it looks like that is possible, but in reference to my original question is there any rule of thumb that one should not exceed when it comes to barrel OD vs chamber when bumping the barrel back? Rustystud help? Thanks

 

[Ackman]

It needs enough diameter at the shoulder to where there's some surface area against the recoil lug. Seems like about .050" would be minimum. I've had a couple Rem. varmint weight barrels cut off and rethreaded/rechambered and there was plenty of shoulder. Winchester has a smaller barrel shank than Rem. and the Win heavy barrel has a much smaller shoulder, so trying to put a Win. HB on a Rem is marginal. For that there are other solutions.

About diameter....A T/C Contender barrel is 7/8" breech diameter and it's plenty for a .223 sized case.

 

[rayjay]

Leave it as a 223. Dies and brass are not expensive.

 

[wziz]

I had the same question as Codywy so I made 2 worksheets to use when figuring barrel strength around the chamber, and at the muzzle for threading breaks, or designing a contour. I can email the Pdf's, don't know how to attach them to this post. I used a CM4140 ultimate strength of 165Kpsi for typical barrel hardness, which is a whole other subject of analysis. Ultimate is where the steel no longer contains the sudden load.,there are lower Kpsi figures where it can stretch and return, or stretch and deform)
These are conservative and simple calculations and don't consider the complexities of concentric bored cylinders, receiver or bolt lug structures, for example, when looking at bolt thrust. Look at Lilja's site for his formulas on bolt lug strength, and Varmint Al's Finite Element Analysis for an idea of how these complex shapes are modeled. Generally speaking, the wall thickness over the chamber, if reamed into the contour taper, will eventually reach an unacceptable margin of safety. Same with the muzzle end, which can be subject to pressures typically 25-30% of chamber pressure.

 

[Preacher]

As long as you have enough room for the new threads, you won't have a problem with to little metal over the chamber on a 700 action.....

 

[butchlambert]

I asked this in response to his post on BR.Com, why do you have to pick up the threads? What would it hurt to have a .300 thread relief if you set it back that far?
Butch

 

[Codywy]

wziz's response was what I was trying to get at. So for example what is the the minimal barrel thickness one should look for when setting a contoured barrel back. I understand of course you have to have enough metal to end up with a decent shoulder. Does one consider measuring from say the interior diameter of the throat to the outside diameter of the barrel at the location of the throat, would this be the right place to pull a measurement from? Is there any rule of thumb or is it cartridge specific based on their individual pressures characteristics?