First, here's an epic infographic that's been floating round the last few days. Click on it for a bigger image.
An AR-15 platform pistol is a pistol, no different than a Glock 19 or an S&W J-Frame revolver. The buffer tube at the back of the gun is necessary for its operation. Here's another infographic that's been around for a while, but is useful nonetheless:
Notice the only difference between the pistol, which can be purchased over the counter just like the proverbial Glock, and the SBR, which requires you to fill out federal forms, include (usually) a mugshot and a fingerprint card, get a sign-off from your chief law enforcement officer in your area (that will change in July, BTW), pay a $200 tax and wait until you receive your tax stamp, which will take months before you can take possession of the SBR, is the stock.
On long car trips I like to travel with an AR pistol, which offers a lot of firepower in a small package. It's maneuverable in a vehicle; I've shot them out to 100 yards and they feature 30-round magazines. I especially like the .300 Blackout, because it is a cartridge specifically designed for the shorter barrel. My set-up will run with either 125-gr supersonic or the 220-gr subsonic rounds designed for a suppressor.
I could make my pistol an SBR by filling out the paperwork (and doing some measurements), pay the appropriate $200 tax, then when my tax stamp arrived — and ONLY after it arrives! — I could put a stock on the pistol. However, because now I have an SBR, which is a controlled weapon, I am subject to all the rules that apply to such a firearm. For example, if I want to take the gun across a state line, say driving from Colorado to Wyoming, I need to notify the ATF in advance of that move (I believe it's a Form 5320.20). In some cases there are states that prohibit SBRs, but not AR pistols.
My contention is the pistol will do about 95% of all the things an SBR will do without the paperwork or the $200 tax. I can shoulder or cheek the pistol on the buffer tube for a longer shot, which is why my buffer tubes have soft foam on them. There are the various "braces," such as the Sig Stabilizing brace or the Shockwave brace, that attach to the buffer tube to make the gun easier to shoot one-handed. Only a complete lunatic would try to explain the ATF rules on braces (see the first infographic). It has been my experience that shouldering or placing the buffer against your cheek to site the gun works just fine.
If I decide to put an angled foregrip like the Magpul on my pistol (see infographic #2), no problemo. If I decide to put a bipod on my pistol, once again, I'm within the law. In fact, I have a Ruger Charger pistol with a bipod sitting on my desk, perfectly legal like. I can even use the bipod as a gripping point, just like a vertical foregrip. However, if I add a vertical forgery to my pistol, I have created another NFA regulated firearm called "Any Other Weapon," which is illegal to own unless — you guessed it! — you have filled out the paperwork, paid the $200 tax and gotten the stamp. If you bought the gun configured as an AOW, you would all of the above, except that the tax would be $5 instead of $200...from the ATF:
NOTE: there is often confusion concerning the tax on “any other weapons.” The majority of NFA weapons are subject to a making tax of $200 and a transfer tax of $200. Many individuals have the mistaken belief that the rate of tax for making an “any other weapon” is $5 because the transfer tax on “any other weapons” is $5. As discussed in Section 4.1.1, the making tax on all types of NFA firearms is $200.Okay...does that make more sense why I like AR pistols???????