A friend of mine, asked me to pull a load, using a screw jag, on the rifle that was loaded with Pyrodex, for five years. After pulling the sabot projectile and breaking up the caked powder, I dumped the powder on a concrete slab and ignited it. It ignited just fine. Cleaned the barrel and found no signs of rust.
There is a complicated chemical reaction when you ignite BP of Pyrodex. Have been told that chemists have a hard time explaining it. What is well know, is the that when ignited, various "Salts" are produced and that is what starts eating your barrel. ....
Once saw a poster in a dentists that had a picture of a slice of cherry pie. The caption read; "Fve minutes after eating me; I start eating your teeth" ..
The same can be said of BP and Pyrodex. ....
Be Safe !!!