There's an article on chunkguns at
http://www.muzzleblasts.com/archives/vol3no2/articles/mbo32-1.shtmlI just looked through Don Davis' Muzzleloading Shooting and Winning with the Champions and what he says is (paraphrasing and summarizing): Load depends a great deal upon the twist of the rifling, usually between 1:48 and 1:72. All will work if you work up a proper load for them. For 1:48 in 50 cal, he used 85 gr of 2F, where in another .50 he used 80 gr 2F. One in 48 twist is accurate, but very temperamental in that a slight load deviation would spread bullets all over, but a 1:66 twist is more forgiving.
So, given you've already got the barrel, the twist has already been chosen for you. But, a 48 cal is so close to 50 cal I'd expect it to have the same general characteristics. I'd start with a tight ball/patch combo and 80 gr of 2F, and then be prepared to do a lot of shooting of 5 shot groups off rest, and varying one parameter at a time (ball diameter, patch thickness, lube, powder charge and powder granulation) until I thought I'd found what gives the tightest group at chunk gun range. I haven't tried it, but if you use the search engine of the ALR site here for 'chunk gun' you might find some earlier discussions that would give you a better idea of where to start with a load.
Good luck, SCL
