Except that it's illegal in many, if not most states to possess an NFA item under state law. They are usually classified as "dangerous weapons" or some such. The exemption that allows us to have NFA items is most often language that says "unless properly licensed" which is interpreted to mean you have an NFA tax stamp.
As I wrote in another post, in Colorado for example it is a felony to possess an SBR, SBS, machine gun or silencer. There is an "affirmative defense" available that the item is properly licensed and permitted. But an "affirmative defense" is something you plead to the court AT TRIAL (usually at the arraignment), not to the cop who stops you and arrests you for felony possession of a dangerous weapon. Under the "affirmative defense" system the police can (and in some cases will) arrest you even if you have all the paperwork on your person, because they CAN. If they don't like rednecks with SBRs or mouthy "sovereign citizens" who give them attitude they have legal authority to hook and book you and let you spend the weekend in jail till your arraignment is scheduled. If at the arraignment (or sometimes another hearing if the DA is feeling particularly pissy) you claim the affirmative defense and present the requisite proofs to support it, your case can be dismissed by the judge. But that doesn't mean you're going to get your SBR or can back any time soon, because the police, if they are prone to this sort of action as some departments are, will hold on to it for as long as possible and force you to get a court order for its release...and you'll be lucky if it hasn't "disappeared" from the evidence room, or been fatally damaged by the time you get it back, if you get it back.
Listen, I'm a retired detective and I can tell you as a matter of fact that there are many, many police officers out there who DO NOT LIKE armed citizens at all, much less ones with SBRs, machine guns and silencers. The closer you get to a major metropolitan area the stronger and more institutionalized the anti-gun and anti-NFA opinions are. I worked with a good many cops who knew the gun laws of the state, including the NFA issues, like the back of their hand and were eager to violate ANYONE they stopped with a gun for SOMETHING they had done wrong, no matter how trivial, sometimes as a matter of department policy set by the administration. And yes, they would arrest you for a forward grip on your pistol.
Federal agents can be just as rule-bound and inflexible, and I've run into Forest Service LE Rangers who are likewise proficient with the federal regs and will bust you for anything they can find or think they have found...like shooting at trees. Yes, it's illegal in some forests to shoot at trees, or rocks, or anything but "paper targets." And if you've seen some of the ad hoc shooting areas west of Colorado Springs you'll understand why, because they look like the Ardennes Forest after the Battle of the Bulge from decades of literally mowing down the trees with gunfire. While it's silly to complain about a few acres among hundreds of millions of acres that are seriously impacted by irresponsible shooters who bring propane tanks and old washers to shoot at, they are usually visible from a forest road so all the gun-hating hoplophobes see them when they are out bunny-hugging and they complain to the Forest Supervisor who then looks for an excuse to shut the area down to shooting altogether. Happened right here with a very old range on Rampart Range Road west and north of Garden of the Gods. It had been used for decades with FS permission and then one day a couple of years ago some idiot negligently discharged his handgun in the parking lot and killed a friend of his. Within 48 hours the range was PERMANENTLY closed and they had erected concrete barriers to keep people from parking. The Forest Supervisor had been WAITING for some incident that would let him shut it down because it's been a trash and maintenance thorn in his side for a long, long time.
So please don't try to tell your grandpa how to suck eggs. This is a big deal and I guarantee you that the local Sheriffs deputies in many places will happily write you up for having a forward grip on your AR-15 pistol, and they will (as an organization) get to KEEP the item. They don't have to involve the feds at all because it's all illegal under state law.
Let's hope so, but the point is that we should all comment anyway, because if we don't, we give up the moral right to complain about what they end up doing.