Here is an intereseting example that actually happened.
I have a friend who inherited approx 150 guns and well over 100,000 rounds of ammunition on his fathers premises. He had to move them to his premises. It was a real problem to transport it.
The Sheriff contacted the ATF to find out what to do. The ATF sent four people in two vehicles to escort the shipment. All Guns and ammo were inventoried prior to transportation and reinventoried at the destination. The ATF did not have a problem with the owner having them on his own premises, but a real big problem with the transportation of them.
The basic rule of thumb - If there is a question about whether a certain weapon is legal to own - Ask the sheriff's department - They will call the ATF if they do not know the answer.
Amazingly enough, some private citizens can own automatic weapons and even things like a grenade launcher. They need a good reason, must be trained in their use, and be licensed by the ATF and the state. Not all states allow this. Private sales of these weapons are not allowed. The ATF will take them if one no longer wants to own the item(s).
Shesrmann