Quote Originally Posted by DesG View Post
I knew a bloke who loaded his ammo and then dry tumbled them. They came out really shiny, but the second shot blew his old 303 to pieces! The tumbling ground the nice granules of propellant into very fine powder high explosive!
Old wives tale and there's no empirical evidence to conclude that tumbling live rounds causes any problems.
You'd have to tumble/vibrate the loaded rounds for a few years to get it to grind down enough, even then it might not.
The MoD did extensive testing on this exact issue as some of their ammo could be subjected to large amounts of vibration in its life before it gets fired. They concluded that it was a non-issue.

If it was factory ammo then I'd be asking more questions of the company, but as it was reloaded there is a bigger chance it was due to a badly loaded round, most likely a light charge. Extremely difficult (impossible) to prove as the round was fired.
I have also heard that commercial ammo is tumbled before it is loaded into boxes to be sold to make the ammo look lovely to the buyers.

A guy on ARFCOM did a 200 and 300 hour tumble experiment and microscopically examined the powder before and after, he concluded there was no change to the powder. The pics of the results can be found here.