I use 777. I also shoot powerbelts. 80gr. 777-295gr copper hollowpt.
I have shot 8 rounds without cleaning. The only thing I noticed was getting the bullet the last quarter inch or so to seat. After awhile you gotta put a little muscle in it the last little bit. I have had no fouling in the bore but a slight accumulation in the breech area. As far as cleanup, I remove the plug and put the barrel end in a bucket of plain water. I then pump a patch up and down. The thing is, as soon as the water hits the residue it is gone. dissolves immediately and with zero smell. I like the idea of using 777 because it is supposed to be non-corrosive, no sulfur. I think that the heavy loads, 120gr and up, leave more fouling because the powder is still igniting and on fire as it comes out of the barrel. I am new to this so I could be off mark, but it is my experience so far. 777 is the only thing I have used but I see no reson to try anything else. My wife hates the smell of rotten eggs, so I'm sticking with 777.