"Tumbling live ammo will degrade the powder and cause dangerous changes in the burn rate."
This seems to be another long lived reloading urban legend that is often repeated, even by respectable sources. It ignores the fact that ammo gets jostled around more in military Humvee's or your standard pickup on rough dirt roads than it does in a tumbler.
Rather than simply accept it as gospel, a few individuals from the website The High Road actually put this to the test more than 10 years ago. They loaded batches of ammo with different powder types (ball, flake, etc) and tumbled 1/2 the batch of ammo for
48 hours in a vibratory tumbler.
They then broke down some rounds from both the untumbled control group ammo and the tumbled ammo. They took close-up pictures of the powder and saw zero signs of powder breakdown (no powder "dust", no degradation to the granules). Then, they chronographed rounds from both groups and found no statistical difference in velocity.
Tumbling loaded ammo had zero net affect on power appearance or ammo function.
https://www.thehighroad.org/index.p...ults-from-tumbling-loaded-rounds.498890/I don't personally tumble live ammo as I don't really see the need, but there is no harm in doing so (except in peoples minds).
