The RMC Flintlock is a nice rifle. I would suggest you try a load of Goex black powder as the basic charge and some Goex 3f or 4f as the prime charge.

That barrel is a fast twist Green Mountain Barrel Flintlock. If it is the one I am thinking of, some of them have a removable breech plug. The older AccuSporter does not. It will probably tune in around 100 grains of Goex 2f and a .430 diameter 300 grain HP-XTP or just a XTP made by Hornady. Start your load development at 80 grains and then start working upward from there.

Be sure to swab the barrel between shots. Use a lightly damp patch with isopropyl alcohol. Swab the barrel is short strokes, working from the muzzle to the breech and then back out. Flip that patch and do it again. Then run two or three dry patches after thatdamp one, in the same manner. Be sure the barrel is nice and dry before loading another sabot.

With a sharp flint and a good prime, you should get some excellent accuracy. I thought about buying one of them rifles with the laminated stock when RMC was selling off the AccuSporter that does not have the removable breech plug, but passed on the deal.