I've left percussion revolvers loaded for months on end with no issues. Recently I tried a roughly eighth-inch "fiber" wads (thick paper really) between the powder and the greased felt wad, the idea being that it would act as a filler to bring ball closer to forcing cone. I'd fired dozens of this load with no trouble, and in fact they were pretty accurate. That's 30 grains 3F, the fiber wad, a felt wad, and .457" ball, with a touch of Bore Butter over the top. Not saying there's anything special about the load. That's not the point.

After cleaning, I loaded two cylinders with the same load for the '58 Remington and put the gun away. About a week later I pulled the gun out and; Weird. The grease I'd put over the ball had been pushed out, and some of the balls were above the cylinder face. This only happened to the one cylinder that was in the spare cylinder pouch. The cylinder in the gun, with the same charges, looks normal. Both were stored with the chamber mouths facing down (not that it should matter).

Anyone heard of such a thing? I could easily push some of the ball back into the chamber with my finger.

My theory, and I can't confirm it until I can duplicate the result, is that I used too much seating pressure, mashing the ball outward against the cylinder wall. The combination of the two compressed wads providing outward pressure, and the ball "springing back" slightly after being deformed in the chamber, resulted in a loose ball with enough pressure behind it to push it out.

It is of course possible that I'm completely wrong.