Building an AR Pistol: What You NEED to Know

All the goodness you can buy.

AR Pistol Build

It has come to my attention over the last couple of months, that a lot of gun guys are scared to build their own AR style pistol. Not scared that it won’t work, or that they don’t know how. These are guys that slap together a new AR in a wildcat caliber and take it to a match the next day. Scared they will get the paperwork wrong, and then take a vacation to Club Fed. So we set out this week to sort out exactly how to roll your own and get it right.

The completed pistol.

Now there are a great many excellent factory built AR pistols. I have reviewed several, and I take nothing away from them. If you have the money and not the time, having one delivered to your local shop is the way to go. But like many of you, I also enjoy assembling my own. (Build is a strong word when it comes to AR’s. They are basically Legos, no skill required.) The best part of making my own is that I get exactly what I want, and often at a reduced cost.

Home Defender 9000, ready to rock n roll.

So, what is the first step with an AR pistol? How you register the lower matters. Yeah, I get it, you could probably put pistol parts on any AR you own, and no one would ever know. But that is especially sketchy with a gun you are likely to have for a self-defense situation. My AR pistol rides in the truck most days and is my weapon of choice in the home. My pistol might always be on me and be the most likely candidate for self-defense, but the AR pistol is a close second. And how could this go wrong?

Odds of you plinking at the range and the ATF guy in the next lane decides to both question your build, and run the serial number back to your FFL and see what it started life as? Near zero. But what if your gun is seized as evidence after a defensive shooting? What if you live in an unfriendly state, and the local DA would like to hang you on something, no matter how justified the shoot? Do you think the powers of Government might be inclined to pour some resources into checking it out then? Don’t forget, Al Capone went down on tax evasion. And that is all.

So it’s best to just do it right. Considering the price of a lower is about $80, it is in no way worth the gamble. All that said, how do you do it right? I was actually pretty amazed by what I found out researching this article. My belief was that a lower that was a pistol had to stay a pistol, and a rifle, a rifle.

10.5-inch barrel, brace fully deployed.

Fortunately, my FFL, MCM firearms, decided to dig into it fully. The relationship between an FFL and the ATF is a lot different than the relationship between a citizen and the ATF. Especially when that FFL has all the classes of license, like create suppressors and machine guns. The ATF at that point is like a friend, who’s job is to help you with compliance. They benefit from the revenue you create and therefore have a vested interest in helping you along the way.

So MCM called the local branch, to clarify exactly what is kosher and what is not. The answer was actually more fluid than I thought. A stripped lower, shipped to the FFL, is registered as a receiver. Not a rifle or pistol. And therefore you can do whatever you like with it. A pistol lower with furniture is a pistol and can become a rifle if you like, and then go back to a pistol. The only one that is stuck is a rifle lower (ie with rifle furniture, like a real stock). You cannot put pistol furniture on the rifle lower, and make it a pistol.

I was actually kind of shocked about the pistol or stripped lower bit. Especially that a pistol can be a pistol, then a rifle, then a pistol again, with nothing required of you. It turns out, this one actually went all the way to the mat. Thompson Center spanked the ATF’s butt red at the Supreme Court back in 1992 over exactly this. As long as you don’t ever put your pistol upper on a stocked lower, you are golden (meaning don’t ever put a barrel shorter than 16 inches on a lower with a stock).

Knowledge in hand, we decided to go ahead and build a pistol this week in honor. The video is mostly about the parts I chose, not a how-to build. Since I already have a video on how to build an upper in 45 minutes, mold your own lower, and build a regular lower, we skipped that part. But, in my opinion, the video about the parts is more informative and useful.

Aero lower.
Upper and lower fit.

Aero M4E1 Receiver Set – The real star of the show! I picked Aero for a variety of reasons, and the price was one of them. Aero is a unique animal in the firearms world, where the price is low, but the quality is on par with ANYONE in the business. I know several high-end custom gun builders that use Aero receivers, which tells me a lot. Aero has also done a lot to advance the cause of home building, and make the job much easier.

Integral trigger guard.

The Lower receiver; not only does it look cool, but it also has vast improvements if you normally build on mil spec lowers. The rear takedown detent spring hole is threaded, which means you only have to put the spring in once. Secure with an Allen key, and forget about it. No more lost springs when you swap buffer tubes, and no more damaged springs when you try to put that buffer tube on to begin with. The trigger guard is integral, which is huge. The easiest part to break during assembly is the aluminum “wing” which holds the roll pin retaining the trigger guard on a regular lower. At which point your lower is trash, as there is no fix. Aero eliminates this possibility entirely. I also like that the E1 series is marked for 45-degree safety, a feature that I have recently learned to love.

45-degree markings

The upper looks fantastic and fits perfectly with the lower. So fit aside, why choose Aero? Because the forward assist and ejection port cover show up factory installed. Both of those parts are a real pain in the ass to do, and not something you are likely to customize. Big kudos for Aero putting them together for you. MSRP $202.48

Barnes Precision makes excellent rifles, many of which I have reviewed on the channel. But fortunately for us, they also sell components.

Nickel Boron BCG

Bolt carrier group– I like the Barnes BCG for two reasons. First, it is Nickel Boron coated. This not only gives it excellent wear resistance but adds inherent lubricity to the metal which helps reliability. If you forgot to lube your gun this week, the Nickel Boron is much more likely to still cycle, due to the metal treatment. The second reason has to do with this being a defensive gun. I know Barnes has exacting standards. The number one part to break in an AR-15 is the bolt lugs, either due to improper heat treat, or mistiming of the cam pin. It is a catastrophic failure, and one I would never risk in a real use gun. This is not the time for a $25 Chinese pot metal bolt. MSRP $210.00

Indestructible Barnes Precision Handguard and barrel nut.

Handguard– I will readily admit, this is a bit of an indulgence on a pistol length build. I love the Barnes handguard, both how it feels and functions. And I know it is as hard as a coffin nail, from doing some destruction testing a few years ago. The Barnes handguard survived things nothing else would. You could get away cheaper here, no question. But, it is an excellent choice. $330.80.

Such a cute little barrel.

Ballistic Advantage Barrel- In a pistol build, the one place I will save money is on the barrel. I don’t really expect to need this gun at the range, and certainly not to exceed 200 meters. I also built mine in 300 AAC, which in my experience is not exactly a stunningly accurate cartridge in any platform. So for the build, I went looking for the cheapest barrel I could find. That happened to be a Ballistic Advantage on closeout. I was going to tell you guys that I consider 4 MOA acceptable in a pistol build, etc etc. But then something stupid happened. Using Hornady ammunition, my clearance barrel ended up shooting ¾ inch groups at 50m. Extrapolating to 100 meters, we have a 1.5 MOA gun. You will never hear me smack talk Ballistic Advantage again. For the price, that is incredible. MSRP $150 (found on closeout for $60).

Perfection in a box
The Gold standard, and one piece. Easiest trigger install ever.

AR Gold Trigger– Since I opted to cheap out on the barrel, you might be surprised I didn’t on the trigger. Since the trigger is the number one way to improve your accuracy, and we didn’t care about accuracy, why? Two reasons:

One, a light trigger is still faster than a heavy trigger. That is basic physics. In a defensive situation, I might need to put a lot of lead in something, stat. A faster trigger is definitely to my benefit here.

Two, mechanical accuracy is relative in a gun like this, but a good trigger still helps with shot placement. Thinking about the shot Jack Wilson had to make in Texas this week, a good trigger still helps in a mixed target environment. Having been trained at hostage rescue, I know this. You might have both a limited exposure window and a small target, you have to hit to save the day. And if you jerk a shot due to your mil-spec 8-pound garbage, an innocent person is still going to die. I’m not in the Army anymore, I will always splurge for a decent trigger. MSRP $279.00

Maxim Defense CQB brace. Pricey, but extremely cool.

Maxim Defense CQB Pistol Brace– Alright, I know, I’m going to get slayed for this one. The Maxim was absolutely an indulgence. You can get by with a brace 1/3rd the cost, and functionally it will be just fine. But the Maxim is so cool! It does shorten the overall length as well, if that is a consideration. And you only have to buy it once.

Serious length reduction, without sacrificing performance.

There you are! If you have been hesitant to build an AR pistol, I hope this helps. Never have so many parts been available, and at mostly reasonable prices. I recommend you get yours ASAP before the ATF figures out a way to reclassify them all as SBR’s.

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About the author: Clay Martin is a former Marine and Green Beret, retiring out of 3rd Special Forces Group. He is a multi-decade and -service sniper, as well as 3-Gun competitor and Master ranked shooter in USPSA Production. In addition to writing about guns, he is the author of “Last Son of The War God,” a novel about shooting people that deserve it. You can also follow him on twitter, @offthe_res or his website,

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  • Greg W February 11, 2021, 12:27 pm


    Can you send me the complete list of items to build the defender 9000? I have a 80% receiver already.



  • John white October 18, 2020, 2:50 pm

    How much cheaper can i build a Ar 15 pistol?

  • Ron Kramer October 5, 2020, 11:44 am

    So – I buy a complete lower. And a 7.5 upper.
    I get them and put a brace on the lower and join the LOW TO UP.
    I have a pistol. (lower has serial and multi on it).

    do I now have to register or do any paper work? I heard RI60 but then others said NO. I did not receive a pistol? (I did not buy a pistol?) I built one and was told I don’t need to do anything. ?? TRUE?

  • Robert Coats April 1, 2020, 8:18 pm

    Clay, how are you man! The house? I’m in Boise and of you need a hand please reach out. Bob

  • Bill hitt March 31, 2020, 3:16 pm

    Aero M4E1 Receiver Set $202.48
    Bolt carrier group $210
    Handguard $330.80
    Ballistic Advantage Barrel $150
    AR Gold Trigger $279
    Maxim Defense CQB Pistol Brace Minimum $349
    Total=$1,521.28 so far!

    Where is the coin saved??

    • Adam S October 10, 2020, 12:36 pm

      There is no coin saved here. When you splurge on half of the parts, it isn’t splurging anymore.

  • Patrick LeDoux March 30, 2020, 2:59 pm


    Please send me the total cost and the list of parts to build this Home Defender 9000.

    I really like the gun you put together and want to make one for myself.

    What Optic do you have on the rail?

    Hope to hear back from you.

    Thank you so much, Clay.

  • Lance March 30, 2020, 1:20 pm

    I love Aero, best bang for your buck. Aero also owns Ballistic Advantage. K think Ballistic Advantage are the best kept secret in AR barrels. They tend to be very accurate and we’ll made. Great Article!

  • Jeremy Haynes March 30, 2020, 10:07 am

    Would have appreciated more of a breakdown in price and a cost of the total build. Only gave the price of a couple of parts.

  • JohnH March 30, 2020, 9:51 am

    I’ve been planning on building an AR pistol myself and wanted to use the Maxim Defense CQB brace. If you were shoulder it, do you think the cheek weld would be comfortable? Or would the buffer tube push into your face?

  • JayC March 30, 2020, 9:42 am

    You forgot to mention the other really nice thing about the Aero Precision lowers – the nylon tipped screw under the grip that tensions the upper-to-lower fit, resulting in no rattle.

  • MikeRoss March 30, 2020, 8:45 am

    “So MCM called the local branch, to clarify exactly what is kosher and what is not.“

    Which is worthless. The only opinions from the BATFE that count are the ones in official letters. Six different BATFE branches can be asked identical questions, and six different answers can be given. It’s hard to believe a gun shop doesn’t know this.

    • Green tip March 31, 2020, 8:30 am

      EXACTLY what I was thinking. ATF has REVERSED ‘AND CHANGED’ directions on AR “pistols” and what constitutes any and all so much, that just about the only thing you can count is the politics and that THEY SHOULDN’T EXIST. They are a detriment to freedom and common sense…

  • Mike March 30, 2020, 3:03 am

    You can buy a complete lower that even has a stock on it and as long as you dont slap a carbine or rifle upper on it, you can just change out the buffer tube to a pistol buffer tube and make it into a pistol. You actually dont even have to change out the buffer tube, you can keep it but the stock has to be removed before you put a pistol upper on or it becomes a SBR and with out a tax stamp, you would have serious legal issues. The key is that that the complete lower is never turned into a complete rifle. On the 4473, all lowers, stripped or complete are all transferred as “other”.

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