Right, muzzle-loaders aren't controlled by the Feds at all - you can have them shipped to your door. But muzzle-loaders are VERY slow to reload and take some serious practice to be able to operate reliably - you literally have to assemble each load as you go with the right amount of powder, the ball stuck in there properly and the percussion cap placed right.
Flintlock is even more annoying
There's one tiny loophole in all this left. You buy a percussion revolver, the sort of thing Wild Bill Hickock was into like the Colt 1851/1861, the 1858 Remington or Ruger's "Old Army" which is a semi-modern percussion "general old-timey replica" loosely patterned off the Remington.
You then get a cartridge conversion cylinder for it, to take light-loaded modern ammo.
In Arizona and likely New Mexico and Texas, you can't find used Colt 1851s in .36cal for love or money. They get bought by Mexicans, shipped south over the border in their original antique-replica condition and then converted to 38Special with cylinders built in small machine shops. The 38Spl is one of the calibers Mexican authorities consider "non-military" and hence the penalties for packing one (IF you're a Mexican national!!!!!!!!) are "lighter" (and many cops will let you slide with a small bribe).
But again, reload times vary from "slow" to "take the whole damned gun apart" depending on how good the conversion is - the Mexican El Cheapo conversions are of the latter type. Useful against one or two muggers maybe, that's about it.