Actually , it depends . Some of the modern guns fall under the FFA and there for must go through an FFL . Sadly it also now depends on what state your sending the gun to as a few now make it a real hassle to send even a traditional muzzleloader without going through an FFL
The Fed has a list of muzzleloaders which fall under the FFA which you can download .
But the vast majority do not need to go through an FFL
Now some are going to disagree with what im going to say next and that’s ok .
Depending on your modern design , the ignition system may in fact predate the 1898 rule .
Case in point , plunger type inline ignitions which actually date to the early 1700’s
As of late , some dealers have become so , no pun intended “gun shy” that they will only ship through FFL’s . IMO that’s just a CYA thing and thus personal choice
Some folks also have suggested that if your modern inline uses rifle or 209 primers that it also must then be sent through an FFL .
I don’t believe this to be true as that ruling was amended .
But if your modern inline is capable of interchangeable barrels to modern center fire , then yes its most lily on the Fed list of required FFL shipment