The mechanism? i.e., as in a science based explanation as to what causes OSR? I too would like to understand it as would many others I’m sure.

Maybe an oversized hard bullet somehow “irons” the bore and displaces metal when the elastic properties of the barrel steel is overcome or exceeded. Maybe because the barrel lands are not able to displace the bullet material because there just isn’t enough space/time to displace bullet material and all things being equal, the barrel loses the battle? Or maybe it has nothing directly to do with the hardness of the bullet as surely high grade super strong modern barrel steel is far and away harder than the brass or bronze bullets with no stress grooves that seem to be the catalyst for this anomaly. Maybe the super high pressure wave that follows the bullet as it travels down the bore causes it?

Who knows???

Certainly an explanation as to “why” it happens would be much appreciated..... In lieu of that, I am smart enough to recognize some of the factors that come together to create it and to avoid it like the plague.... once burned, twice shy.

Interestingly enough, for the longest time, nobody knew what caused chamber ringing until some French dude decided to mess with things and set up elaborate controlled experiments with a pressure vessel. Now any reloader with half a brain knows not to stoke a large cavernous case with overly light charges of slow powder and they know to ensure that fillers are used in conjunction with reduced loads and appropriate powders. Chamber ringing happens due to a pressure wave of expanding gasses violently colliding with the bullet base..... who would have thought that expanding gasses could move chamber steel like that?