Plus any gun made before 1898 that is magazine fed is not considered an antique. This includes lever actions, bolt action rifles (that aren't single shot), etc. If it is centrefire, it must be an obsolete cartridge that is no longer in large scale production and if memory serves me right it has to be over . If it is rimfire it must be any cartridge other than the .22 family. There are many many more restrictions that I'm not even going to go into...
Blah blah blah...the point is nobody is going to make a large scale production single shot antique style rifle that was originally chambered in some obscure and now obsolete cartridge...unless you can get your hands on an airsoft musket (why!?) you're out of luck and should just buy these guns through the proper channels.