MasterSergeantA isn't wrong about constructive possession, but I think people worry about it a little too much.
First, just to clarify, since people often get this confused: constructive possession is a legal term meaning that you possess something that you don't actually have in your hand. Constructive, in this instance, is a legal term that is the opposite of "actual." Where people sometimes get confused is in thinking that it has anything to do with "constructing" a short-barreled rifle or a machine gun. It doesn't. A good example of another type of constructive possession might help illustrate this.
If you have a baggie of cocaine in your hand, that's actual possession. If you have a baggie of cocaine under your driver's seat, and you're in the car, that's constructive possession. You don't "actually" possess it, but you legally do.
So if you have a non-registered AR15 lower receiver, and you put a short-barreled upper on it, then that's ACTUAL possession. But if you have a non-registered AR15 lower receiver and a short-barreled upper sitting in an otherwise empty gun safe, but they're not assembled together, that's CONSTRUCTIVE possession of an illegal short-barreled rifle.
Where it gets fuzzy is that all AR15 uppers and lowers are theoretically interchangeable. So if you have one SBR-registered lower receiver and one regular ole AR15 lower receiver, and you have a variety of uppers for both, some folks get concerned that the ATF will come along and say, "AHA! You could assemble both your lowers as SBRs, but only one is registered! You're going to jail!"
Here's my opinion, though: don't worry about it.
First of all, there has to be a reason for the ATF to be looking at you. If you're a regular person who doesn't get into any sort of criminal trouble on a regular basis, the ATF isn't ever going to be peering inside your gun safe. They just aren't allowed to come inspect your home without a warrant, and they have bigger fish to fry anyway. (If you're a interstate cocaine trafficker, you might be at risk, but you probably don't register your SBRs anyway.)
Second, I think the only situation that a decent criminal defense attorney couldn't get you out of is one where you've actually assembled an illegal SBR (short upper on a non-registered lower) and that's ACTUAL possession, not constructive possession anyway. Or... if you had only a non-registered lower, and had a variety of both short and long barrel uppers sitting in the gun safe with it. Then they would (perhaps rightfully) ask "Why have a short barrel upper if you don't have a registered lower? You're clearly planning to assemble them together, illegally, some day even if they aren't assembled right now." THAT is constructive possession in a nutshell.
But if you have unassembled AR15s of both the registered and unregistered variety with both short and long barrels available, logic does not dictate that you will eventually assemble an illegal weapon. If there's a way to do it legally, then "innocent until proven guilty" protects you by logically assuming that you're only going to assemble them as legal firearms.
In short, as long as the parts you own can be assembled into a legal configuration, rather than ONLY being able to be assembled into an illegal configuration, I wouldn't go worrying about constructive possession.
Aaron