Tumbling loaded ammo will cause the powder to physically degrade and result in dangerous pressure increases.
This is one of those “truths” that’s been repeated over time to the point that it’s considered gospel and at first glance sounds reasonable—classic BullShooters territory. The theory is that the vibration of the case tumbler will break down the construction of the “kernels” of powder, causing different burn rates. The deterrent coating on the powder will also “rub off,” causing the powder to burn more quickly which will raise pressures. A quick Internet search will produce plenty of examples of this “truth."
The Flawed Reasoning
Loaded ammo travels around the country on trucks, powder spends weeks bouncing and rocking across oceans in barrels coming from places like Australia, and belts of linked ammo spend hours shaking violently across the sky in military helicopters, all of which would create similar vibratory forces as a case tumbler.
The Expert Deferral
Logic alone isn’t enough when it comes to harnessing propellant gases mere inches from a shooter’s face, so we asked experts at two of the World’s leading powder and ammunition makers. I spoke to the Chief Ballistic Scientist at Hornady Manufacturing and the Head Ballistician at Hodgdon Powder and asked for their professional opinions. Both agreed that this is a myth devoid of empirical data.
"Powder is hard, it doesn’t change shape from any reasonable amount of vibration,” said Hornady’s Dave Emary. “This notion that you can wear deterrent off of the surface of the powder is a myth, it is impregnated into the powder grains. You can’t knock this stuff off."
Both scientists felt that tumbling was a safe practice within the bounds of reason.
We are calling BullShooters on this one. While extended tumbling could, at some point theoretically cause a problem, a reasonable amount of tumbling to clean up loaded ammo is not dangerous.