I tend to not shoot much smokeless in original trapdoors, but all of my shooters over the years have experienced at least some smokeless. Tolerable accuracy--the basic 4-5 MOA that most trapdoors in good condition will do with the equivalent of BP issue ammo--can usually be reached with bullets sized to .459. Remember though, when you work up a load with a .457" bullet and decide to increase the diameter, it is a good idea to drop back to starting loads and work up your load to match the desired velocity.
The next step up in accuracy from here is to select a bullet with a nose that fits the bore. Take one of your bullets and press the nose into the muzzle. If it drops in, it is too small. If the nose of the bullet slides in easily, it is still too small. You want a bullet that you can force in by hand, and when removed shows 3 lightly engraved markings from the lands.
The third step up is to size the bullets to the largest diameter that will chamber. Make up dummy rounds with bullets sized .459", .460", .461", and larger if you can. Chamber the dummy rounds til you find the one that won't chamber, then go to the next smaller diameter.
I'm still testing loads in my current rifle (Model 1873 with 1879 sights). I've only got about 400 rounds of smokeless through it, all smokeless equivalents of the M1882 Rifle loads (500gr bullet at ~1,300 fps). When testing I prefer to shoot 10-shot groups at 300 yards. My averages so far:
.459" bullets: just under 14" (eight 10-shot groups)
.459" bullets, nose fit to bore: 11" (ten 10-shot groups)
.461" bullets, nose fit to bore: 10" (ten 10-shot groups)
I haven't tested the rifle with duplicates of the M1898 Rifle loads (original smokeless loads, 500 gr bullet at ~1400 fps). If I were going to shoot smokeless loads in the long range match, this is the load I'd use. My aged and decrepit carcass much prefers BP carbine loads!