Be carefull, and read the statute carefully. In many states ownership is lawful *only* if the federal tax stamp is obtained. I know that's the way it is in Texas. From the Texas Penal Code:
§ 46.05. Prohibited Weapons
(a) A person commits an offense if he intentionally or knowingly possesses, manufactures, transports, repairs, or sells:
(1) an explosive weapon;
(2) a machine gun;
(3) a short-barrel firearm;
(4) a firearm silencer;
(c) It is a defense to prosecution under this section that the actor's possession was pursuant to registration pursuant to the National Firearms Act, as amended.
So in Texas, you're still not able to own them without jumping through the Federal hoops.
A quick search of Arkansas code reveals a similar requirment for federal registration. It's in called the Subchapter 2. Uniform Machine Gun Act of Chapter 73 Weapons of Title 5 of The Arkansas Code. Just go here and search for "machine gun". You'll get the bad news. Possesion with "offesive or aggressive purpose", which is presumend w/o the registration, will get you minimum 10 years. The same as you'd get if you robbed a store at knife point.