Sundance,
I have not shot 777 in my C&Bs yet. I have 14 at present. However, a 15gr load would be no more powerful than a 20gr FFFg BP load. Where you might get into trouble is if you tried a full load of 25grs (by volume) and really rammed the ball down hard. I have nearly filled the chambers with Pyro P and rammed conicals hard and gotten a pretty stout load from that. I could tell after the match that I had to tap the wedge in a little deeper so I must have caused a few parts to "give". If you were to fill the chambers with 777 and stacked on a conical I think you'd be taking chances. Over the years I have on numerous occasions put too much powder (3F) in the chamber and had to really crank on the rammer (even whittled some lead with a knife) to seat the conical or ball and had no sense that the gun was overloaded when it went off. However, I do know of a Colt Walker that blew apart with a charge of 777. The barrel sailed down range leaving the frame & cylinder behind. The cylinder pin (arbor) snapped off at the wedge cutout.