AR pistols are the new kids on the tactical block. Here's what you need to know.
I know what you're thinking, "I thought AR-15's were rifles. How can there be a pistol?" and for the most part you're right. AR pistols are AR-15 platforms that have a shorter barrel and a "pistol" buffer tube that does not allow for the attachment of traditional stock.
The National Fire Arms Act of 1934 (NFA) defined any rifle with a shoulder stock and barrel less than 16 inches in length and 26 inches overall to be a "short barreled rifle" (SBR) and required a citizen to fill out a BATFE Form 4 and pay a $200 NFA Tax Stamp. In 1934 $200 was a lot of money and was a back-door gun control measure, among other things.
The AR pistol was developed to get around that NFA restriction since tax stamp waiting times are still several months. That's a long time to wait for a piece of paper that says you can possess a firearm that you have to buy in order to fill out the paperwork. So while you wait for your tax stamp the SBR sits in the safe at your local gun store that you bought it from.
The AR pistol allows the shooter to have the size, weight and maneuverability of an SBR, but without the NFA regulations. It's a a short AR-15, but with a different buffer tube and no stock. Pistol barrel sizes are usually seven inches or 10.5 inches, which gives the shooter a vast amount of flexibility in close-quarters shooting, making this a great home defense firearm. It also saves on storage and transportation due to its small size.
Thanks to Sig Sauer for producing their Pistol Arm Brace shooters can have the look and feel of an SBR, but without the cost and hassle of a tax stamp. When I was working in a gun store we carried completed 516 pistols from Sig Sauer and they, ironically, came with a certified letter from the ATF saying that they were pistols and didn't need a tax stamp. Whether, you buy a Sig Sauer 516 pistol or just the arm brace, you have an AR pistol. Though, the original intention from Sig Sauer to is put your arm through the brace and hold it like a pistol.
The legacy of the AR pistol is not without its flaws. Such a short barrel length means that there is also a very short gas system. Check this out for a further explanation of AR-15 gas systems. The shorter gas systems cause the firearm to be over-gassed. This means the gasses cycle faster than in a normal rifle, which causes more recoil and louder operation. Pistol systems require an H2 or H3 buffer to offset harsh operation, but it only helps a little.
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Over-gassing will degrade the components of the pistol faster than a normal length gas system. If you are going to go for a pistol build, I recommend buying higher-grade parts than what you would have gotten away with in a rifle-length system. I recommend a Mil-Spec 7075-T6 aluminum lower receiver versus a commercial 6069-T6 aluminum receiver. The Mil-Spec on is nearly twice as hard on the Rockwell scale and will hold up better than the commercial. This means that you will be paying a premium in order to extend the life of your pistol, yet you will have your firearm longer.
In conclusion, the AR pistol is a great way to have an SBR without the tax stamp and the wait. Even though it is a pistol, be sure to check out your local and state laws regarding AR pistols. I only quoted federal law.