Questions about firearm registrations

[Creeky73]

In the process of tinkering with my first AR, I am of course finding myself with a slew of parts left over and am strongly considering building a second gun with them. Since it would be an "extra" I am thinking about making something fun but not very practical, like a pistol AR, but first I need to acquire a new lower. Should I buy a lower that is designated for pistol use, or can I set up my old lower as the pistol even though its serial # has it as a rifle? Also, what makes an AR a pistol, besides being limited to the short barrel? Is it illegal to put a stock on them? Is there a tax stamp involved like on SBR's? Also, if I buy a new lower for my primary rifle instead of the pistol, can I use the barrel that is registered to the old lower on it? Basically I have no idea how the whole process of registering these guns works, especially if you are building one from scratch. Any help is appreciated.

[Army Chief]

I believe the prevailing school of thought here is that ATF takes the position that "once a rifle, always a rifle," so the critical question is this: how is your receiver registered? If you don't buy/register it specifically for pistol applications, then the resulting weapon is in jeopardy of being ruled an illegal SBR.

More to the point, an AR pistol cannot possess a stock, nor can you put a conventional VFG on it without changing the classification of the weapon; something that many an HK SP89 owner discovered when he started shopping for a MP-5K VFG.

All of this notwithstanding, pistol ARs are registered and transferred as any other pistol would be, and do not require tax stamps or NFA approval.

AC

[Creeky73]

yes, what I am gathering is that since my current lower came original on a registered rifle, I had better not use that one for a pistol configuration. I have been reading a good deal about this on AR-15.com and most of those guys are saying that a new stripped lower is not sold as being either rifle or pistol, so technically you can use it for either, but it would be best to have a lower marked for pistol use just to avoid any question. Since it is not legal to have a VFG on one, would the presence of a 4-rail handguard bring up any question? I assume that since you can legally mount lights and laser devices on conventional pistols, you should be able to mount them on these, right?

[TehLlama]

The last detail is that dropping the pistol upper onto a stocked lower creates an SBR, a big no-no.

Some states treat pistol magazines differently as well - keep that in mind. For example, if I were to bring my SBR upper from home out to where I'm stationed, I'd have to get a pistol lower, and then Cali style bullet button-ize it with a 10rd magazine.

[Liljake82]

This is interesting. I have a friend that is into TC contenders and he has the same problem. IIRC he says a pistol reciever can be turned into a rifle but you can't turn a rifle reciever into a pistol.