I have used 777 in some BP cartridges. In my .50, I had high pressure signs with very moderate compression. Be very careful with the seating of the bullet. I used an card wad under the lead bullet.
Daryl has warned of immediate pitting and frosting of the bore of new barrels by one shooting session with Pyrodex. Not sure about the 777.. I do note that the velocities with 777 are higher than most BP substitutes.