See how to clean empty brass shells. – Do not try this with live ammunition! – It’s quite simple, really, even with these very dirty .30 caliber shells. They were stuck to the cotton ammo belt, found inside a rusty .30 cal. MG ammo case at a farm in the Ardennes (or so I’m told). Anyway, after having the cartridges demilled with an unloading hammer by a professional, the heads got separated from the casings, but everything was still very dirty.
The secret: citric acid. You can buy it at your chemist. I used about 60 grammes for 1 liter of water and left the shells soak for about 6 hours. Then you still need to dry them off and polish with a soft cloth, or maybe even with brass polish, but the result is really nice.