Replica "Antique" cartridge-using firearms and the internet
Chuck Michel has a memo on the issue. A cartridge-firing modern replica of a pre-1899 firearm can still be an antique, and thus not a "firearm" for GCA purposes, if it is chambered for cartridges which are no longer manufactured in the US and which are not available in the "ordinary channels of commercial trade." The question is whether the internet has made rare cartridge sufficiently available to where antique status may be lost for some replicas.
Not sure how much of a problem this might be, as the replicas I can think of use cartridges that are available thru ordinary channels. Maybe .50-90 and .50-110? If "ordinary channels" means via local gun shops, those would qualify, but be at risk since there are some companies offering loaded cartridges.