Hi Ed - first thing I'd do is to slug the barrels of both rifles and record that data.

Also- put a tight patch on a jag and measure the twist yourself. It's easy. - Push the patch into the bore an inch to a couple inches, then put a piece of tape around the rod at the muzzle and one at the handle - black pen mark at exactly bottom dead centre on the tape at the handle. Push the rod into the barrel until the black mark on the paper is exactly at the top dead centre on the rod, ie: the rod has rotated 180 degrees. Put another piece of tape on the rod at the muzzle, front edge contacting the muzzle as before, around the rod. Pull the rod out and measure between the edges of the tape that touched the muzzle, both times to get 1/2 the twist rate. Multiply X 2 = the rate of twist for that barrel. Do this with both rifles. If you mark the top dead centre with a rifle length barrel, not a carbine, you can shove the rod into the muzzle until the pen mark it top dead centre on the rod - ie; 1 full rotation.

I was under the impression that all Trapdoors, from 1873 to 1884, were all 22" twists- perhaps the later ones were increased, I do not know this as fact. Your results will be interesting, so please let us know.

You'll probably want your cast bullet to be .001" to .002" larger than the groove diameter. You will want to use something like SPG or Lyman's Black Powder Gold for lube. The bore is important only if you was using a long nosed bullet like the #457125 Lyman mould, with the bore riding nose. The nose usualy has to be fairly snug in the bore, for that bullet to shoot really well.

Some bullets have shorter nose sections, like the gas check Lee 500gr. mould, but if the nose is 'just' touching the lands, that's better then being sloppy, even on that bullet. The RCBS 500gr., 400gr. and 300gr. moulds have very short noses that do not ride on the lands, but the long bullet shank that must be seating inside the case takes up a lot of powder space.

Lee has an intermediate bullet with a flat base - a 460gr. which might suit both rifles?

If you are needed a .459" or .460" bullet, the Lee moulds might not cast large enough.
In that case, the cheapest mould that will most likely work, is the Lee 400gr. HB mould, but the bullets will most likely have to be case out of 20:1 tin to lead to allow expansion of the base to fill the grooves - it's all experimentation.

Few years back, a fellow won the Quigley match at Hefley Creek Rendezvous in BC, with an original Trapdoor 1884 Model. The contest was offhand, 10 shots at a steel plate some 750yards up on the side of the hill - actual range, perhaps 600 to 620yards - just a guess. The plate appeared to be 6' X 4' rectangular. He hit it 6 times out of his 10 shots with the rifle's original Buffington battle sight and blade front sight. I think his bullet was the 500gr. Lyman bullet, #457125. Some of those moulds are good while some of them cast undersized bullets. It's a crap shoot.

Further note about moulds that I just thought should be noted here is that the Lee mould bullets do not have very large lube grooves and will not hold enough lube to keep the fouling soft. Your bores will most likely foul excessively and probably lead badly as well.

I would have a look at Buffalo Arms for moulds that are designed for shooting black powder. They will have large, square lube grooves, not narrow, rounded ones like smokeless powder cast bullet moulds have.