#Flamethrowers should absolutely be regulated under #NFA1934 as incendiary "destructive devices", as codified in 26 USC 5845 (f):
' The term “destructive device” means (1) any explosive, incendiary, or poison gas (A) bomb, (B) grenade, (C) rocket having a propellent charge of more than four ounces, (D) missile having an explosive or incendiary charge of more than one-quarter ounce, (E) mine, or (F) similar device; '
operative words here: "incendiary" and "similar device"