I'm with Dan. I'd use real BP, and in 2F granulation. I would not under any circumstances use bore butter. Tests and results from other shooters shows it builds up in the bore. Black powder gives closer shot to shot pressures and therefore better accuracy potential.
: Currently, I am using LehighValley lube, sold by trackofthewolf. It has proven to be the cleanest shooting, most accurate lube so far and I've tested hundreds of different lubes over the years, including bore butter. It was poor.