4" @ 50yards, I am surprised.
The side lever and overall rifle, is really "NEAT"!
Bottom instead of left side extraction device, again "NEAT".

Buffalo arms might have a mould, in a heal bullet closer to the requisite calibre?
Other mould makers do produce custom moulds.

The old 158gr. HB round nose might be a better bullet, however you would need to find older CIL ammo, I suspect, or a mold to cast them.

A .38 Case, fired in my .357 Ruger, comes out at .378" diameter.
What diameter do the .38's fired in your rifle expand to?