FWIW:
I tryed the IMR 4198 in my Snider and have ignition problems.
Was very funny as a couple of cartridge keep the gases from the primer inside so make a funny noise when I take the bullet out.LOL.

For BP load you can try a trick of britishmilitaryforum but with FFg intead of 777:

Brass............ Converted Magtech 24 ga. to .577
Case length... 1.975"
Primer........... CCI #209 shotgun primer
Main charge... 50 grains Triple "7" ffg
Filler mix........ 20 grains ffg B.P./20 grains C.o.W.(mixed well)
Wad............. One .70"dia milk carton wad
Lube............ One 1/8" lube cookie
Bullet........... .590/310 grain Round Ball pushed into neck
Over all length 2.265". This round is loaded tight, no compression and no loosseness.

Looks good, if you use only BP you can get 115gn into this case.
JMHO
Martin