An antique is not a firearm UNLESS it fires or has been altered to fire any cartridge available in the ordinary commercial channels of trade. The ATF doesn't normally enforce this reg, but it's there.
IOW any 45-70 or 44-40 or 30-40 cannot qualify as a non-firearm antique, no matter WHEN it was made originally. This was a convenient loophole that allowed folks some freedom, until the DC weenies plugged it. But, like I said, they don't normally enforce it.....unless they see an opportunity.....
Regards, Joe