Can't speak for BP, as I've never used it. 777 seems slightly easier to clean than Pyrodex (but I only used Pyrodex once - about 30 loads). I can't speak from experience, but all I've heard is that Pyrodex is slightly more corrosive than 777 and both are worse than BP. I clean my ROA as soon as I'm home, about an hour after finishing shooting, with hot soapy water and it's straightforward.
I've just done a bit of measuring - the empty chamber is approx 32mm deep, 24gr of 777 fills 16.5mm, the wad is 4mm and I measured a .457 ball at 11.65mm. The ball seats approx 2-3mm below the cylinder face (I'll double check that when I shoot tomorrow night) so everything is snug, with powder and wad compressed by a couple of mm. I don't use grease and have never had a flash over. I fill the chambers from pre-measured glass vials and make sure there are no loose powder grains anywhere else. It works great and is certainly more accurate that me
Shooting it is here.... https://www.youtube.com/watch?v=EX6iKfznbs4