Ok, I have been searching through the topics and have come up with several questions for my first build.

#1: From what I read, all you need is a stripped "virgin" lower to build an AR pistol. Is this true? It doesn't have to have been purchased as a pistol receiver? Hmm...wouldn't this make it legal for a person under the age of 21 to indirectly purchase a pistol?

#2: Is it conspiracy to violate the NFA to have assembled lowers hanging around the house when you have an AR pistol? I only ask because due to the nature of the ar-15, it would take only several seconds to pop off your pistol upper and attach it to one of your other assembled lowers.

Quoted: Ok, I have been searching through the topics and have come up with several questions for my first build. #1: From what I read, all you need is a stripped "virgin" lower to build an AR pistol. Is this true? It doesn't have to have been purchased as a pistol receiver? Hmm...wouldn't this make it legal for a person under the age of 21 to indirectly purchase a pistol? #2: Is it conspiracy to violate the NFA to have assembled lowers hanging around the house when you have an AR pistol? I only ask because due to the nature of the ar-15, it would take only several seconds to pop off your pistol upper and attach it to one of your other assembled lowers.

that is a very good point  "loophole"

This is my understanding:
1.  Unless you are under 21 and actually build a pistol then there is no problem.  If you buy the lower for a pistol build and procede to build a pistol and are under 21 then you have violated the law.  You may have found a way around the law by getting the lower but you have still in the end broken the law when you build the pistol.  Just because you legally bought the lower does not resolve you of responsibility if you get caught after you build it into a pistol. Sort of like being under 21 and buying ammo that can go in a rifle or pistol and you tell the clerk it is for a rifle, knowing full well it isn't.  You may have gotten the ammo but you still broke the law.

2.  If you have rifle lowers and pistol lowers, and you own rifle uppers and pistol uppers, you are good to go for the most part.  Now, if you own one pistol lower, 7 pistol uppers, 6 rifle lowers, and only 1 or three rifle uppers, then you are really crossing the line.  So long as you keep the rifle lowers matched with an equal number or more of rifle uppers and keep the number of pistol uppers in line with number of pistol lowers, there is no problem.

The law is not so much what you might be able to do but more along the lines of what you can only do.  Yes, by owning a rifle and a pistol you MIGHT be able to put the pistol upper on the rifle lower, but unless you do it there is no violation.  The ONLY part is if the only way they can be assembled is a violation, such as having pistol uppers and no pistol lower or pistol uppers that far out number your number of pistol lowers while owning rifle lowers that do not have matched rifle uppers.