This is an appeal from a jury conviction for knowing possession of an unregistered firearm in violation of 26 U.S.C. Â§ 5861(d). The alleged firearm is known as a ground burst projectile simulator, model number M115A2, and is used by the military in the training of infantry troops. It simulates incoming artillery fire and exposes the troops to the accompanying sound effects. This device was seized from defendants person when he was about to board an airline flight from Cincinnati to Milwaukee and was detained at the security checkpoint by a guard. Defendant Dalpiaz was subquently convicted of unlawfully attempting to board an aircraft while carrying a concealed and deadly weapon in violation of 49 U.S.C. Â§ 1472(l). This Court affirmed the conviction challenged on Fourth Amendment grounds in United States v. Dalpiaz, 494 F.2d 374 (6th Cir. 1974). Shortly before the jury verdict had been returned in that case, Dalpiaz was indicted on the charge which is the subject of the present appeal.