In the last few years i have noticed bullet creep in some of my reloads. I never attempt to chamber a round that has resistance . Hate spilled powder in the action and such. I did have a stuck round on a ar that i couldnt morter loose. Gingerly drove the bullet into the case and then poured oil into the muzzle , left it over night and drove it out the next day.
If in the R the bullet had crept forward , jammed in the rifiling, and some smut got into the extractor cut, that ould lock up the thing like fort knox.
What touched it off i dont know and never want to find out first hand! Possible a spark from the rod if steel, glad he still has his fingers.
