Thank you Curl

I think for those of us who haven’t had the ‘privilege’ if we could call it that, of seeing OSR for ourselves, your explanation is quite simple and clear. You used the term “deform” in your post and then also have confirmed that it seems in some cases barrel joining ribs have been loosened from the possible stretching effect on the barrel metal.

Trying to think outside the square, I put forward a theory in an earlier post that perhaps OSR is not a deformation (if I use your term) of the barrel metal but a heat effect on the barrel surface. I happily accept this theory is not the answer from the explanation that you have provided.

Next question! In your experience or opinion, once OSR has appeared does it get worse over time i.e. if carrying on using the same loads that possibly caused OSR to appear in the first place, is the next stage over time likely to be loosened ribs?
I suppose most users pull up and review the bullets they are using but if there are examples of loosened ribs then these users have obviously carried on oblivious to the OSR developing on their firearm, unless of course the OSR was so severe and loosened the ribs with the first few rounds using the ‘offending’ bullets.

I suppose I’ll just stick to the old Mauser 404 and worry like hell every time I loose off a shot that the bolt doesn’t blow out from my ‘lengthened’ standard action. Then again I have the fantastically accurate cast bullet load for it which should see the barrel last for ever.