Sabots normally need a faster rate of twist- like 1:28. You ask about conicals that have bands and are long. They will normally not stabilize in a 1:48 ROT, much less in any ROT that is slower. Those long bullets need a fast ROT like 1:28, or 1:20.
The 3 ROT barrels you have mentioned were designed to shoot mainly RBs, but can shoot very short conicals, like the Lee Precision,R.E.A.L. bullet.
You sound like someone who doesn't have a clue what a RB that diameter can do on game, and are working with a bit of information about bullet formance you gained from being around modern high speed cartridge guns. Those are apples and oranges compared to BP ballistics. When you get to .54 caliber and larger bore sizes, the RBs are so haavy that the Normal result of shooting any game is for the ball to completely penetrate the body, and leave some of its energy on the ground behind. Its an exception to the norm to find any part of a RB still inside the body of game.
With a .50 caliber, you can expect the RB to completely penetrate deer sized game, because they are thin-skinned and have very small, soft bones. Most deer are shot well within 100 yds- in fact, most are shot under 50 yds. At those ranges a 50 caliber RB will go through most deer.
So, there is no need for you to be searching for huge conicals and barrels to shoot them. If you want to hunt bear, or Moose, buy a .58, or .62 caliber rifle. Those RBs will bring down any bear or moose that walks the earth. Those balls may be a bit of an " overkill" on deer, if there can ever be said to be such a thing.