Which T/C are you shooting? Some of the traditional style rifles have a slow or round ball twist. Some have a "compromise twist' of 1" in 48" and should stabilize a slug or a sabot. If memory serves, the R?B barrels are so marked.
The fact that you are getting tight groups at fifty yards with no sign of keyholing seems to indicate that the rounds are being stabilized. However,
my experience with sabots is pretty much limited to watching others try and work up good loads, so maybe they don't begin to wander until they get a little farther out. The suggestion to try fffg is a very good one and you could even try ffg---I've had .50 calibers that preferred one over the other. I have two new .54s and I anticipate trying several loads in each before I get it right. And it's unlikely that they both will wind up liking the same load. But that's part of what makes front loaders so much fun, at least for me.