If you ever notice it, all gun laws or should I say most gun laws on the federal level are based on interstate commerce. It has been stated in opinions that since every gun has been out of the state at sometime or a part of it has then it is interstate commerce. But, that gets me to wondering what if....lets say you build a reciver out of metal dug from the ground in your state and formed into a receiver in your state, then built into an NFA gun without ever leaving your state. Would that somehow be interstate commerce ? You better be prepared to prove it isn't. Not being able to is a sure conviction.
But, back to the issue. I would say you could get anything from nothing to 10+ Years on multiple trumped up charges. I heard of a case where a man was charged with 2 counts for 1 gun. Why, because they considered the DIAS to be an MG and the gun to be an MG and it was therefore 2 violations. They can also hit you with AW violations. Then again on top of Federal charges, the state also may have laws against NFA guns. Notably, Montana (>7.63mm)and a few others don't. Vermont has only minimal fines in the range of $10. But, some other states can send you away on state charges for as long as the Federal sentence or possibly more.
But, it all comes down to where you are. The police in the city where I grew up were the laziest I've ever seen. The crime stats claimed it had a low crime rate, but that was only because there was a near-0% arrest rate. Unless you did something serious they didn't want to take the time to do the paperwork. I've seen people walk for Breaking & Entering, Robbery, etc. Then again, one neighbor is still in prison on DD charges for throwing 2 pipebombs at a house. So at least they do something when it comes to attempted murder. But, interestingly, he wasn't charged with attempted murder, but rather Making a "Unregistered" Destructive Device. He was arrested 5 years ago and is still in Prison.
But, if you know the Police or you are in an area where they don't go alot then you are less likely to be arrested unless the ATF gets wind of it then prepare for a BBQ. For example, in the mountains of N.C., you can do alot of stuff and get away with it. Why, because it takes 30 Minutes to drive up the mountain and the nearest town was another 20 minutes down the road. If you are somewhere that is inaccessible then, violating the law isn't as likely to get you in trouble as long as you keep your mouth shut. But, in a city especially with either nosy neighbors or a big mouth you are looking at alot of trouble.