There would be no cartridge. It is a replica of a muzzle loader. If he has the skills and machinery to build the gun, he should be able to build a mold for bullets. The nipple cap could be formed from shim copper.
Somewhere there is info on the chemicals that causes the spark. The trick would be to determine the amount needed to ignite the black powder without having the copper becoming a projectile or several projectiles. BVB
If I feed mine properly, will it grow up to be that big?