Pedantically, it would be considered an "antique firearm". Still a firearm, but basically unregulated
according to the law antique firearms are not firearms.
https://web.archive.org/web/20050313043608/https://jpfo.org/...
Three years later, in 2007 the ATF decided differently, "Upon further review, we have determined that the string by itself is not a machinegun..."
https://everydaynodaysoff.com/2010/01/25/shoestring-machine-...