A leading cause has been detached base wads. This goes back at least as far as the '70s, when Remington's All-American hulls had plastic base wads that occasionally came loose after several reloads.

Recently there have been several barrels destroyed by the same problem from today's hulls. I think (but am not sure) that the new, Winchester poly-formed hulls, reloaded, have been the biggest offenders. I also know of one that occurred with a factory load.

Fred