ML.

I'd give BlackMagic3 a try if it wasn't for the difficulty in aquiring it. I have a components supplier in Rochester, NY that I by bulk from, and he might be able to get a couple of pounds, don't know.
Another option that I thought about was Hodgdon's "Select"........?
It reads as though it would perform real well, but you never see anyone writing/talking about it much.
The Harvester Sabots look like winners also. As far a sizing goes, regarding bullets, that's where cast shines. I can size bullets down to the exact diameter (+sabot) that will fit the bore. Usually, when sizing a cast bullet to a specific revolver, oversizing it can ruin the bullet and degrade it's accuracy potential.
Oversizing (slightly) regarding muzzleloaders, if necessary, shouldn't be an issue as there's no direct contact with the bore.

Can't wait for the gun to get here and get some benchtime. Got to start gathering powder & sabots.

Take care,
Bob