Can't help with the .54 cal as I've only shot the 200gr.REAL in a .45 with a 60" twist. I sized them to .451 and lubed with Lyman BP lube in my lubesizer. As the .45-90 in the Winchester High Wall and Model 86 had 60" twists, I figured they should shoot OK in the slow twist ML. The 45-90's shot a 292gr. as the lightest bullet, up to 340gr. they also weren't very accurate, with 6" being a normal 5 shot group at 100yds. The were supposed to hold 9" at 200yds. I firgured this was due to a too-slow twisand that a shorter bullet, the REAL 200gr. would be much better- it is. My flinter will hold 2" with them, the same as RB when there is no wind.
: The REAL's hold to my flinter's RB sight picture right to 100yds when using 80gr.3F.(REAL) There is no recoil to speak of. With the .440 RB I shoot only 60 to 70gr. The reason for sizing them was merely for lubing. As cast, they came out with the bands measuring .456 which would really engrave them. Due to the thin bands, unsized, they'd not be hard to load. With my sized bullets, they slide down easily and obturate perfectly to engage the rifling when fired. In the 42" barrel, with 80gr. 3F, the velocity shold be around 1,700fps, about the same as a factory .44 mag would from a rifle. When you consider the 44-40 rifle's velocity was around 1,300fps with a 200 gr. bullet, the REAL bullet is a worthwhile projectile for the .45. Mine weight 201gr. average with lube.
: I haven't shot anyting with them, but my buddy, also shooting them in a 60" twist GM barel, reports that to 100yds, they slay deer immediatley. He's also shot deer in the ribs with .440 RB and found the deer dropped in their tracks, whereas with the REAL bullet, they walked to a total of 20yds after being hit. There was no mad dash, as heart shot deer will sometimes do when shot with a modern rifle, up to the '06.
: I would suggest that in a slow twist, that the lightest REAL bullet be tried first as the shorter the bullet is, the better it should shoot. The longer it is(heavier) the less stable it will be, especially after it hits something. An unstable bullet won't penetrate in a straight line and will follow the easist path through the animal instead of smashing the bones and tissues assunder.
Daryl