I'm putting together a .36 underhammer and was wondering how many others shoot one now. It will be replacing my .50cal Hawkens as my rabbit rifle so planned on using a .35 round ball and FFFg black but what charge works for you? It will have a 1:48" twist so would it stablize some 105gr round nose cast bullets I have. They are so short that they are almost a round ball anyway.

I shoot 30 FFF with a 360 RB out of my buggy rifle. I like big balls and thin patches. My barrel is an old Montana with a 1 in 48 twist.

I bought some of the gray sabots and have been shooting some of them. Even made some RN cast bullets myself. Nothing oustanding with the 87 [?]grainers. The 105s might be too long.

NBHunter who are you buying your underhammer from? I would like to have a .36 BP to go with my other BP guns. Let us know how well it shoots when you get setup.

I've pretty much settled on Green Mountain for the barrel. The rest I'm making at home. Got the action "roughed up" now working on the parts.

NBHunter,
You can't go wrong with a GM barrel. Their trophy room is an impressive place. I have fired their .36 drop-ins (same bbls, just in finished form) when I worked for them some years ago. A quick visit to their website tells me they are not making drop-ins in .36 anymore, but they still sell .36 blanks, so you're good to go.
As for shooting them, I have never fired one that was not at least capable of MOA, often much better. You are going to have to experiment a little to find which powder/load your rifle likes best. I used .350rb's (I still have the mold) with OxYoke prelubed patches and I played with loads from 25-50gr by volume of FFFg BP and up to 60gr of Pyrodex RS. The one they let me take home and play with liked the heavier charges of Pyrodex. I would stick four of those 5bull .22 targets on the holder at 100yds, shoot a bullseye in each of five, swab the bore with a little BlackSolve, run a jagged patch to remove the crud and soak up the solvent, then shoot the next target. I did not experiment to see how many times I could shoot before loading became difficult or accuracy dropped off, but it was definitely match accurate on my Renegade stock, so I stayed with the system that obviously worked so well.
As to the 105gr cast, if they are cast from pure lead and the lube grooves are deep enough to hold sufficient lube, they should work fairly well with your twist rate. Are they hollow base designed for revolvers? If so, fill the base with lube, too. I suspect that the prb's will be more accurate, though only experimenting will tell. I like T/C 1000+ Bore Butter, as it shoots clean and really helps to control fouling with both BP and Pyrodex, but crisco will work, too, it's just messier. I would avoid animal fat based lubes as they go rancid in a relatively short time.