I'm back after a long time away. If I had a George Schalck rifle, I'd be jumping up and down for joy, or, at my age, at least grinning a bunch. Please tell us about the breech-loading part. How was it converted? Can you provide photos, please.
If a .38-40 WCF case fits the chamber, I see 3 options:
A: Try it with fixed ammo. First, remove the false muzzle & put it someplace safe. Then try some sort of ordinary .38-40 revolver bullet, something around 160 to 180 grains, cast very soft, and maybe 30 grains of FFFg powder and a card wad.
B: Breech seat with the same bullet. As an experiment, try the same revolver bullet breech-seated & well-lubed. Same light powder load. I'd try just a gentle push & tap to get the bullet into place.
C: Muzzle load with a cloth patched round ball. My picket rifle has conventional rifling, but the bullet base is swaged round and it came with a circular patch cutter. Put a wood plug in a fired 38-40 case to use as a guide. Put the plugged case in the chamber. Load the round ball using the false muzzle. Push the ball down the barrel until it hits the plugged case. Remove false muzzle!! then remove the plugged case and reload with a case nearly full of BP and a card or felt wad.