100 grains of any powder is not likely to be fully burned before the bullet leaves the barrel, resulting in a big muzzle flash and perhaps a dangerous pressure spike.
There are several variables that need to be addressed before a real answer can be given. What grain conical or sabot? Folks think alot of powder and a 250 or 275 grain bullet is going to give them maximum performance. For the most part that is BS! If you want "Oomph" down range at the target, then a heavier bullet and less powder will do the job better. A 250 grain bullet out of a 50 or 54 caliber just doesn't have the inertial mass to carry long distances. On the other hand a 45-70 using 65 grains of powder and a 500 grain bullet was used to for one shot kills on Bison,( with one shot) at 300 to 500 yards.
A 250 grain bullet is too darn short to stabilize and overcome air drag beyond 125 yds.
IMHO, if you want to increase your killing power on game, change to those new 400 grain bullets in a 50 cal, and reduce the powder to 60 grains. Even at that it'll be a wrist bruiser.
Triple 7 is burns with greater pressure than black. 100 grains and a heavy bullet will increase the pressure substantially.
Hogdon's once had the pressures published in their brochures for various loads. It isn't there anymore in the new brochure I just picked up. It does have the pressure for some metallic loadings. 80 grains of Pyrodex pellets (a 50 and a 30) in a 45-70 behind a 405 bullet has 27,200 CUP thats about 50 % more pressure than I would shoot in a black powder gun.
There have been some rumblings here an elsewhere that 777 should not be used in bottle neck cartridges because of high pressures.
I do think you are correct in assuming equal volumes of powder will burn faster in a 54 than in a 50. although the difference in speed really isn't that significant as compared for instance between a 50 and a 58. The faster the burn, the higher the pressure. That is why 3fg is used in smaller bore guns, 2fg in medium size bores and 1fg in the large ones.
