Larry must be shooting sabots and light for caliber bullets in his fast twist 54. It certainly isn't optimum for caliber bullet length and weight. The reason 50 and 54 caliber rifles are not seen in BPCR and weren't used but very little, if at all, for paper patch bullets in muzzleloaders is because short for caliber bullets are not stable at much distance. Period. For bullets in calibers that large, indeed any caliber, they should be long for caliber to be truly stable. One needs only look at what the long range shooters use, myself included. You don't see any 300 gr. bullets in 45 caliber winning any matches any more than you see 240 grain 40 caliber bullets winning any matches. Nor do you see 150 grain, 30 caliber bullets being used. The same was true of the 40 and 45 caliber paper patch bullets back in the 1850-1900 era. A careful reading of Ned Roberts "The Muzzleloading Cap Lock Rifle" should be required.
As I understand it the standard load for the M2 Browning MG is a 710 grain bullet at 2700 fps. The Barret is similar. Obviously we aren't going to drive bullets that heavy, that fast out of any muzzleloader, not a real muzzleloader anyway, but the fact remains that a bullet of heavy weight, 600+ grains, is desired for a 50 caliber for longer than average distance or paper patch shooting. A 600 grain bullet at 1400 fps generates significant recoil. For 54 caliber the bullet would necessarily be even heavier. Quite possibly in the realm of 900 grians. That's about 8 bore round ball weights. Drive 900 grains of bullet to 1100-1300 fps and see what the recoil is like. It will not be a pussycat, even in a heavy rifle. India and African hunting literature is replete with accounts of such rifles and smoothbores being used. That information isn't difficult to find.
WGR, your idea of reaming out a 50 and rifling it to 54 is interesting and sounds like something I would like to do. The 1-28 rate of twist seems to me adequate but I'd do some serious research first. I also have an interest in paper patch muzzleloading, conical shooting rifles, altho I've never owned or fired one and neither time nor funds allow me to pursue it now. That's another project that comes under the heading of "some day". I would highly recommend you find a copy of Ned Roberts book, mentioned above, and digest everything he says about the paper patch. Even if you decide not to do the rifle the reading is very interesting and enlightening.
The 54 idea sounds like a lot of fun.....but it seems to me some decisions about what you want to do with the rifle and how accurate you want it to be at what range need to be made.
As modern as some so called muzzleloaders have become they are still light years away from the abilities of the golden age of paper patch shooting, cap lock muzzleloaders. Those with fine aperature sights, false muzzles and near perfect triggers and locks. All average rifles still stand in their shadow.
Vic