This might be a stupid question but what is a tax stamp and why do you need one.
A short barreled rifle (SBR) is a rifle with a barrel length of less than 16 inches (40.6 cm) and overall length of less than 26 inches (66.0 cm).
It should be noted that the addition of a shoulder stock of any type to a handgun with a barrel of less than 16 inches constitutes an SBR, and such a modification can only be performed by a class II manufacturer or an individual who has filed and had approved a BATFE form 1 for the manufacture of an NFA regulated weapon.
(8) The term ''short-barreled rifle'' means a rifle having one or more barrels less than sixteen
inches in length and any weapon made from a rifle (whether by alteration, modification, or
otherwise) if such weapon, as modified, has an overall length of less than twenty-six inches.