Sprinkle it out on the lawn.
I deer hunted with a buddy several years ago. He was using his Knight MK85 with Pyrodex. (I warned him) and I was using one of my Super 91s with Swiss 3F and one of my 496gr Trashcans. We had both fouled our barrels before loading, then placed a "finger cot" over the end of the barrels.
We hunted in damp conditions for 3 days. He got sick on the second day, and stayed in bed at the hotel we stayed at. We had left the rifles loaded, (but caps removed) in the cab of my truck the entire time since it was very cold out. On day 3, we went out in the morning and towards noon, it really started to rain hard. Knowing he wasn't feeling well, I decided to call it a bust. We unloaded the rifles before we left our hunting grounds by firing them into the ground. Before he fired his off, I noticed something odd at the crown of his rifle. I asked him to look at it, and sure enough, it was orange from surface rust. Looking inside the barrel as far as we could without actually pointing the rifle at your face, we saw the same. He hasn't used Pyrodex since.