OSR is an acronym for "Over Stressed Rifling."

Barrels on double rifles are relatively thin. If you shoot a projectile that is too hard, such as a solid copper Barnes X bullet, the radial force of the bullet in the barrel; in particular, the radial force of the bullet against the lands, can be great enough to deform the barrel. The result of this occurrence can be seen as a faint spiral pattern, corresponding to the rifling twist, which appears on the outside of the barrels. This pattern is only seen when viewing the barrels at a shallow angle, such as looking down the length of the barrels from one end.

There have been reports of ribs and solder joints being loosened by the stretching forces imparted on barrels in this manner.

As you can tell from the "OSR" threads, there is some debate and controversy as to what actually causes this malady. In my opinion the Barnes X bullet is a shoo-in.

Whether banded solid copper bullets such as the North Fork or GS Custom can be safely used is a question that remains to be answered.

I hope this is a clear explanation.

Curl