I don't have the time or inclination to research this topic as to federal law or Georgia's law, but it does raise some interesting questions:
1-- Can you buy a modern inline-ignition muzzle loader that uses magnum shotshell primers (instead of percussion caps), with a synthetic stock, stainless steel barrel and action, and topped with a laser-range-finding scope and still call that a "replica" of a pre-1898 antique weapon?
No Form 4473, no background check. Just buy it for cash, anonymously, no questions asked, at any sporting goods store?
2-- If the answer to the above is yes, that black powder muzzle stuffer is not a "firearm" under federal law, then can you cut the barrel down to 14" on it and put a red dot optic on it for a close-quarters "in the brush" hunting gun or home defense gun? (Without involving the NFA or paying $200 for a stamp)?
3-- Can you buy a muzzle-loading double barrel shotgun (if they even make such a thing) and cut the barrels down to 12" for home defense, without needing to comply with the NFA or pay a $200 tax?
4-- ATF says a felon can own an "antique" gun, including a modern replica of an antique muzzle-loading black powder weapon. But will ATF still view the modern gun as a "replica" if it's a heavily modified version, with a different kind of stock (thumb hole or pistol grip, maybe? How about a side-folding stock?), different sights (let's say an Aimpoint red dot sight), and a barrel length of only 18" (which is HALF the barrel of what most traditional black powder rifles had, back in the day).
5-- Can a GEORGIA convicted felon (or first offender probationer) have a muzzle-loading replica of an antique or a real 118+ year old antique? (I think NOT, because unlike federal law, Georgia's law on felons with guns has its own broad definition of "firearm" which basically says anything that uses fire to shoot a projectile, and there's no exception for antique guns or their replicas.) (See Code section 16-11-131(a)).