Ben,
I'm using nitro for black loads with excellent results as described by Ross Seyfried. Using Bell brass,IMR 4198, 17 gr of dacron, a wad about the size of a soft ball, and a 600 gr bullet from a NEI mould I'm getting 1765 FPS and 1 1/2" groups at 50 yds. The rifle is a Hollis underlever hammer gun, 577 3". I found I had to use mag primers to avoid hang fires rather than standards, probably due to the case size and the amount of powder involved. Tried using Goex black years ago but couldn't get enough in the case to regulate the rifle at the correct velocity. Given the choice I would think the 3" would be the way to go.