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I had to screw cut a Tikka for a sound moderator yesterday which involved removing the barrel from the receiver so into the press it went, on went the action spanner and a long bar and it started to unscrew very easily. Surprisingly so in fact...

This was the result. Someone has previously re-chambered the existing barrel from .223 to .222 and to do this they set the shoulder back which means the barrel now has an undercut that is approaching equal to the length of the supporting thread. The barrel bears a current proof mark stating .222 and it was only when I spotted the .223 Rem at the underside of the barrel out of sight that I started to suspect something was less than perfect. My guess is the rifle was submitted for Proof with the stock in place, the Proof house inspected it, checked head space and test fired it without issues so they stamped it up as OK which to me just goes to show a Proof mark does not mean everything is good and yes, it is 'Only' a .222 however it still has a maximum chamber pressure of 50,000 psi.

Ovenpaa wrote:My guess is the rifle was submitted for Proof with the stock in place, the Proof house inspected it, checked head space and test fired it without issues

More than likely they didn't even do that.

Oh dear thats not good.

So i take it its not 1 1/2 times the dia any more?

Some of the Steyr Mannlicher match rifles only have that much thread , the unthreaded section is a dead fit in the receiver. I think it's a bigger thread from memory M28 x 1.5 , but only the rear 1/3rd is threaded. There's also a safety ring in the breech area but that appears to do little.

The use of a register or even two as in the case of the CG M41 is fine however a huge undercut as in the image is pretty grim in my eyes.

I am a Chartered Mechanical Engineer and was involved with the design of structural joints for many years. If you carry out a Finite Element analysis of a long threaded joint, the stress distribution is concentrated at one end. As a screwed joint, the one in the photograph is fine. However, the joint also transfers pressure from the barrel to the receiver and the gap means that it cannot do this effectively in the unsupported length. The key issue becomes the thickness of the barrel. Without the previously mentioned FE analysis, i would be unhappy about it. Proof means that it took the pressure once: it is not a way of inspiring confidence for the future.
Fred

Eh. Bit of JB weld to pack it out and she'll be right. :)

Proof doesn't mean they check the quality of the workmanship, only that the rifle survived the two over-pressure rounds fired at that time without apparent failure.

Whether those two proof rounds bring the rifle so close to the working limit of the materials that the next service pressure round could cause a failure is a matter for debate.