I don't see in the original post that RM ever stated that he is or was using a black powder substitute.
This gun was a "gift" from my uncle to my brother that he acquired a number of years ago. We got it, scrubbed it, lubed it, and it sat happily in a temperature controlled environment until Thanksgiving. Odds are that gun never saw a cleaning patch til we got ahold of it, so theres no telling what powder was used and for how long the gunk of that powder had been working on the inside of that barrel.
RM