"Hand grenades are regulated under the National Firearms Act (“NFA”), a federal law first passed in 1934 and amended by the Crime Control Act of 1968. The 1968 amendments made it illegal to possess “destructive devices,” which includes grenades. (26 U.S.C. § 5801.) There’s no doubt that a live hand grenade designed for military combat fits within the law’s provisions—non-military people may not possess them."

http://www.criminaldefenselawyer.com/resources/criminal-defense/weapons-firearms/is-it-legal-own-hand-grenades