The original Colts used gain twist rifling which increased the twist as you got closer to the bore end of the barrel. This allowed the soft lead to deform more easily to fit the lands and groves with out building a gap and slop in the bullet which would allow gas to escape and the bullet to, shall we say, rattle around and not fit the rifling as well. Perhaps using a softer alloy lead might help?
The percussion cap guns were also generally used with a lube of some kind on top of the ball or bullet after the bullet was loaded, usually to prevent flash over, but it also tended to seal the cylinder from weather but it also lubed the bullet as it accelerated through the barrel as a side benefit.
Modern replica versions of these guns do not have the gain twist rifling as far as I have heard. Too bad.
I do have a replica 1858 Remington .44 with gain twist rifling, but most replicas don't. Mine is a target model.