Daryl, here's my take of how this occurs:

Woodleigh's test, which involved driving bullets through barrels, showed that with some bullets and some barrels, the barrel bulged as the bullet passed, then returned to it's original size. The 'egg is a snake' example that 9.3 gave. If the steel's elastic limits are exceeded, it will not quite spring back, and this fatigue can be cumlative.

If I take a high pressure round, say a 300win( chamber pressure well above the bullet's yield), with a typical bolt action countour (very thich at the breech), I believe the bullet will be swagged down to bore size shortly after leaving the chambers, and OSR probably won't be encountered regardless of how thin the subsequent bore tappers to.

If I fire the same bullet in a lower pressure round (well below the bullet's yield) and through a barrel with a much thinner countour at the chamber end, it may not swag down as much, as the barrel may 'stretch' to allow passage even at the breech end. When this less-reduced bullet reaches the thin end, stretching exceeds the steel's elastic limits.

This is just my theory on what is happening, nothing more. I think we read material property data on various metals and then misapply them in some cases. Yes, steel is indeed harder than copper, but I can also easily split a thin steel tube by inserting a tapered copper rod and gently rapping on it if the proportions are right. Pressure from my mallet is low, the steel is harder, but the taper acts as a lever and the hoop stress skyrockets. Someone with a better mechanical engineering background than I could explain this.

Lastly, I still do not understand how the grooves in barnes bullets "give the displaced copper somewhere to go" if the shank between the grooves is still larger than the bore diameter and being engraved itself! I understand the grooves mean less materials must be displaced, but thier idea in the previous sentance would only work if the shank between the grooves was LESS than bore diameter.

My idea for a test was originally to turn down the last few inches of a bolt action barrel to .010" or .09" and polish it to a mirror finish, but I think it would be better to contour the entire barrel appropriately and fire a cartridge in the 30,000 psi range.

Ideally, you could take an unturned blank, cut it in two (so each half has exactly the same bore dimensions), and chamber one piece for, say 300W and the other for 30-30 with typical bolt and double contours respectively, then fire the same monolithic bullets. I believe the bullets from the first barrel would be VERY slightly smaller than the later, but wouldn't know how to measure such minute dimensions accurately.

One last piece of complete speculation, but I presume all of the barrels with OSR 400 mentioned, being on doubles, had cut rifled barrels. I wonder from a materials standpoint if the act of button rifling a barrel pre-stresses the steel in a manner similar to firing a monolithic and are subsequently more resistant to such damage? Similar techniques are used in some manufacuting processes.

This is all just my 2 cents, take it or leave it. I use cast lead in my bore rifles and woodleighs in my nitros and am happy.

Bob