IIRC, Pope's false muzzle barrels were also supplied with a loading rod, mould and boolit starter. The boolit was pushed through the false muzzle to the length of the loading rod, and because of the change in pitch of the gain twist, the boolit stayed put. A charged case was inserted behind the boolit, and may or may not have had a cork wad over the powder in the end of the case.
Modern schuetzen does away with loading from the muzzle and relies on breech seating the boolit (forcing it into the rifling by mechanical force) and then inserting the charged case behind it.
I owned a square cornered, 44 action Stevens in the 47 pattern with Pope false muzzle barrel with numbered false muzzle and boolit starter. I never had the mould, and the rod was missing, but IIRC, the barrel was marked for the proper length of the rod so that it could be duplicated if lost or broken. Mine was a 38-55.
What you folks are wanting to do can be done without a false muzzle. Insert a cork or vegetable fiber wad in the muzzle and then push a bore sized paper patched or grease groove (will need to be soft) boolit down to just ahead of the leade. Blackpowder will cause the boolit to upset and take the rifling.
Bottleneck case? I suppose that it might work, but something like a 7-08 could be expected to foul terribly, not to mention having a problem cleaning the cases after shooting BP. Pouring a case full of lead and drilling it to bore diameter to the flash hole would make for a small powder chamber, but it might work OK with smokeless.
Turning a Ruger #1 45-70 into a 45-120? No thanks! Try a 500+gr tapered paperpatch boolit just seated into the case over a charge of BP and you'll soon see why. Mine would shoot ragged hole groups at 100 yards when loaded that way, but recoil was brutal.
Oh yeah, Pope wasn't the first, but became a master. I believe that it was Schalk from Pottsville Pa who made the first false muzzled breech loader, and possibly a on Ballard at that.