It’s a short barrel rifle.
A short barreled rifle is defined in the US Code, Title 18, Part I, Chapter, Sec. 921. as follows...
(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.
Wow, I did not know that. You think the ones on Gunbroker that say no FFL needed are ATF traps? There are several.