When I hunt with a side lock I fire 3 caps with no powder before loading. I have never found any rust or pitting when the load is not fired and left in the gun for a prolonged period of time.
I just fired a round that was in a caplock pistol today that had been loaded that way. That load had been in the pistol since November and when I cleaned it today the barrel looks brand new and shines, no rust.
I do not recomend you do this but it works for me. I have actuallly left a load in untill the next season that was loaded this way and still had no rust.