ok, even though I'm not a huge fan of guns I think restricting people to muzzle-loaders might be a bit excessive.
The idea stems off of the hunting seasons between bows, muzzleloders and semi-automatics; bow hunters get to hunt first, then muzzleloaders, and finally semi-autos.
As such, I don't think it's excessive. The tax only applies to the ammunition anyways, as such people can still buy themselves semi-automatics. (We could possibly also remove taxes on guns since it doesn't impede killers much, if someone is bent on killing others money is conpletely no-worries) Said people can also still buy bullets for these guns, but albiet at a higher cost.
Because bullets aren't reausable (Okay, to an extent they are), there is money to be made off of bullet taxes that could benefit the governent or something in some way.
And finally, gun owners often buy guns for the sake of owning one. Sure they might want to go to the shooting range and use not-muzzleloaders, but think of it as getting a glass of some good wine. Besides muzzleloaders are completely different shooting experience from semi-autos.
We could also just limit the hypothetical tax solely to weapons that could easily kill many in a short amount of time: automatics, high-capacity magazines, etc.