I tried R.E.A.L. bullets, both 200gr. and 220gr. in my 60" twist, 42" GM barrel on a flintlock rifle. I used Lyman BP Gold for lube and found I could shoot continually without any fouling problems. The bullets engraved quite nicely when loading, on 2 of the bullet's 'bands'. I used a short starter, but likely because it is automatic to my loading regime. I think perhaps choking up on the rod until the bullet was started, might have allowed loading without the starter.
Accuracy wise, they shot about 2" at 50 yards off the bags. This was when patched round balls were giving me sub-1/2" groups at the same range. I used the same loads I was using with patched round balls. 75gr. 3F and 85gr. 2F. The fouling did not build up and I easily loaded a web-patched round ball afterwards- no wiping was necessary.
Years ago, we found the Maxi-balls lost stability on impact when used on Moose. You could not trust a Maxiball to travel in a straight line through the moose- Straight line penetration was not possible, it seemed. Perhaps they would work fine on deer, but in my opinion, they are not only the worse shape as they seemed to collapse into their grooves instead of expanding, but could not be trusted to penetrate in a straight line - which MUST happen. Hitting one of those 3/8" thick ribs would turn them 90 degrees in any direction and a perfect broadside shot wouldn't even go through one lung, let alone both as planned.
I only bring this up to show what the Maxiball did in a 48" twist. They usually gave pretty good accuracy- on paper. On moose they were a total failure.
I was afraid the R,E.A.L. bullets might do the same, especially from the 60" twist on the GM barrel. Would they work on deer- maybe, but then, so would a patched round ball. .44 to .48 calibre Longrifles were very popular in the ML period for a reason - they worked & with round balls.