I decided I wanted a .45-70 muzzleloader some years back and took a Lyman GPR and sent the barrel off to Hoyt to be relined to .458 with a fast twist rate. Of course, the problem was, the breech and nipple arrangement was rated for the roundball pressures, and not those generated by heavy doses of black powder normally found in the heavy black powder cartridge rifles, due mainly to projectile weights.
Hoyt advised me that it would be all right, if I didn't fall into the trap of trying to make it a .45-120-500, because the flame jet out the nipple would burn out nipples pretty quickly.
So, I stuck with 350 and immediately ran into stability issues. The bullet was too short for the twist rate, which was really designed for the long 500 grain bullets. Back to Hoyt for a reline. It came back with a 34" twist and shoots very well now, with 70 grains of FFg. So, in essence, I have a .45-70 muzzleloader, but with limits to how heavy my bullet weight can be, unless I go to titanium nipples, and then there's still the issue of breech thread strength. I'm satisfied that I won't have the plug come back and take the top of my head off at my load, but it also won't be a 600 yard shooter due to the relatively short bullet profile.
Dan