MAKING A BP PISTOL -
IMHO it might depend on where you live. Some hard-nosed District Attorney might read the law straight out as "You can't cut a rifle barrel shorter than 16 inches and a rifle can't be reduced to less than a 26 inch overall length," to paraphrase the general federal law. State laws may be even more restrictive, but cannot be less.
He might or might not differentiate between a breech loader and a muzzleloader. I'd either check with local law enforcement or buy a kit to make a totally legal BP pistol.
Several states do regard muzzleloaders as weapons, IIRC.
Again, JMHO!
GrayBear