Among the millions of military grade weapons in civilian hands are many older, but still lethal (if well maintained) weapons. The U.S. government periodically upgrades it's list of older weapons it considers "Weapons Classified As Curios Or Relics Under 18 U.S.C. Chapter 44." These weapons can be bought and  sold with fewer restrictions. Some you can still get ammo for, while others use ammunition not manufactured for many decades. Some of the more interesting "Curios Or Relics" so listed are;

Armalite AR-15, .222 Remington or.223 cals., produced by Armalite.
Australian, Austen MK I and MK II Submachineguns, cal. 9mm, mfd. in Australia prior to 1946.
Austrian, Swartzlose machinegun, model 1905 and 1907/1912, cal. 8mm.
Austrian, Steyr-Solothurn model MP-30 and MP-34 submachineguns as originally mfd. by Steyr-Daimier-
Benet Mercie, machine rifle, model 1909 (French and American manufacture), caliber 8mm Lebel or .30.
Browning automatic rifle (BAR), model 1918A2, mfd. by New England Arms Corporation prior to 1946.
Bren, light machineguns, any model, any cal., British Commonwealth manufacture.
War II. British Model 1903, 4.5 inch howitzer.
Chinese mfd. copy of the U.S. 60mm mortar mfd. prior to 1946.
Colt, Monitor model machine rifle, cal. 30-06, commercial version of the BAR, 18" barrel, vertical pistol grip, Cutts compensator, mfd. by Colt.
Degtyarev RPD Light Machine Gun, 7.62 x 39mm caliber, mfd. in the Soviet Union, the People's Republic of China, or the People's Republic of Korea (North Korea).
French, model 1897 75mm breechloading cannon of French and American manufacture, in 1899 or later.
Finnish, Tampelia (original), model 1938, caliber 81mm mortars.
German, assault rifles selective fire, produced during 1941 through 1945, in caliber 7.92mm Kurz (7.92 x 33mm).
German Flak 30, 2cm anti aircraft machinegun.
German MP41 submachineguns, cal. 9mm parabellum, mfd. by C.G. Haenel, Suhl, Germany during WWII. (Original WWII German production only).
German, 15cm Nebelwerfer, 41 (150-mm) Rocket Launcher.
Maxim, machineguns of German manufacture, all models and cals.
U.S. 76mm Tank Cannon, Models M1, M1A1, M1A

It's apparently now safe to clean out of the garage all those World War II souvenirs great grandpa brought back. You won't get arrested now. You might have some strange people show up for the garage sale, though.