300AAC Blackout Pistol Build

Disclaimer: There is no intention by the author to make incorrect/false statements; I may know more than the average person but I am not a firearms “expert” by any means so please check every statement and claim I make here before accepting it as fact.    We all make mistakes and there is always someone who knows more, we should all be eager to learn from them (anyone reading this please do not hesitate to inform me where I am incorrect).  Also, many states don’t understand what the word “infringe” means so for many this build may be illegal so be sure to know your state laws, regardless of how stupid, immoral and unconstitutional they may be (I am looking at you NY, NJ, MD, CT & CA).

I decided to build a completely new AR15, I wanted to make it as compact as possible while avoiding the regulation heavy SBR (short barreled rifle) paperwork exercise and 9 month limbo.  I have fired 5.56 AR pistols as short as 7.5″ and many are ridiculously loud and breath fire (the powder is still burning when the round exits the barrel and the shorter barrel means the exit is closer to the explosion).  Barrels that are 10-10.5″ long are the shortest one can typically go without the pistol becoming impractical while using the same loads one would use for their rifle.

I already have an AR15 rifle chambered in 300 Blackout and I am a big fan of the round. It is a .30 caliber round coupled with a 5.56 case that is cut down (by 10mm); the neck gets resized with a very slight taper.  Looking at the picture below from left to right, three 300BLK (7.62X35), one 5.56NATO (standard AR15) and 7.62X39 (typically fired from an AK47).


It fires a rifle bullet but overall the cartridge acts very much like a pistol/rifle hybrid round. It was developed by AAC to be a short range round; with the goal that it would excel when being suppressed while minimizing the changes needed to the M4 platform to implement it.  Because of these  characteristics I knew what I wanted what the barrel of my AR to be chambered in.

Because of its OAL and use of the 5.56 case; the lower, upper, BCG and magazines used with 300BLK are the same as almost any AR15 chambered in 223/556.  But the 300BLK is a slightly different animal; the bullet is 2-4X’s the size of a 556 with a shorter case so less volume is available for powder when compared to a 7.62X39 or 308 Win.  When reloading 300BLK I use the same powder I would use to reload 357 magnum.  I did some research and found that an 8″ barrel in 300BLK would be as short as I should go to balance out my desire to minimize size and maintain practicality.

I purchased a new blemished stripped lower (the lower is what is stamped and considered the firearm) because it was significantly less expensive, the imperfections were unnoticeable until pointed out and after a few days at the range it will all look the same anyway.  Note:  When buying the lower specify that it will be used to build a pistol.  As I understand it; a stripped lower can be used to build a pistol or a rifle but once it is used build a rifle it can no longer be used as a pistol (legally), an SBR is different and won’t cover them here.  I purchased a standard MIL-Spec lower parts kit and a pistol buffer tube assembly.  This is all that is needed to build a pistol lower assembly.  I added a set of YHM take-down pins to replace the ones in the standard parts kit; they are designed to make it easier to break down the firearm (see closeup pic further down) as well as a Hogue rubberized grip and SigTac SB15 stabilizing brace (more on that later).

lower parts pile

After a little work this is what it should look like.

assembled lower

Be careful, there can be some tricky parts and if this is your first time it is more than likely that during assembly at least one small part (detent pin) will be launched by one of the springs into the corner of your basement/garage never to be seen again.  I suggest spare parts kits as well, they can be had on Midway or Amazon for $10-15.  Roll punches and a brass hammer are also recommended.  There are exhaustive instructions and tutorial videos available on the internet so I am not going to rehash them here.  Some basic tools, a few specialty tools and basic mechanical aptitude is all that is needed.

Here is one of the YHM take-down pins installed.

yhm take down pins

SB15 brace

As you can see I also have the SigTac arm brace.  It looks like a stock but it isn’t, it is designed to straddle your forearm and be tightened by Velcro.

Sig provides a copy of their determination letter from the ATF.  It is also legal (at least at this time) to “shoulder” the brace.  A link to the determination letter regarding this specific issue is provided as well.

Determination Letter regarding the Sig Sauer SB15 brace not being a stock. (click HERE for PDF)

sigsauer determination letter

Determination Letter regarding “shouldering” of an AR15 pistol as well as “shouldering” SB15 brace.


To see a copy of this determination letter go HERE.

The upper assembly includes the upper (also an inexpensive blemished version) with ejector dust cover and forward assist already installed, an 8″ barrel (1/7 twist), rail, flash hider/muzzle brake, gas block, tube, BCG (bolt carrier group) and charging handle.  I purchased the barrel from Aero, a DPMS BCG, a knock-off Noveske muzzle cone, a Vltor handle and the rest are components from YHM.

upper assembly parts

Assembly is pretty straight forward, instructions are everywhere on the internet and many of the components manufacturers offer installation instructions on their websites.  The main issue is to follow the torque guidelines to avoid damaging your components.

barrel install

barrel installed

gas block installed

rail installed

fully assembled upper

Now you can install the BCG and charging handle, pop out the take down pins and fully assemble the pistol.

pistol without sides and afg

I am also adding an angle fore-grip and F/R MBUS’s from Magpul.

magpul add-ons

Determination Letter for AFG can be found HERE.

300BLK pistol

To remain legal DO NOT install a vertical grip to the rail, this is one of the features that would make it an SBR.  The ATF has determined that angled grips are not vertical grips and can be used on pistols.  A copy of the ATF’s determination letter is available at the link above.  The ATF has ruled that “shouldering” the SigTac brace does not violate any laws or make the pistol an SBR but to make sure not to violate the law it is best to not modify the brace in any way whatsoever.  As an extra measure I am going to write to the ATF regarding these issues so that I can get a personal reply and have a concise determination letter addressed directly to me.

brace strapped to arm

cue 21 Pilots~Guns for Hands.

I have also included a cost breakdown of this particular build for anyone interested; I think it is a very reasonable price.  I found my components for good prices and took my time sourcing them; for anyone doing this I cannot guarantee those prices can be matched by the vendors you choose but it shows a quality AR15 pistol can be built for less than $1000.


The overall length of the entire firearm is 26″ and quite a sight to be seen.  After seeing the muzzle cone installed I am considering switching the carbine length rail for a mid-length one that is about 3″ longer.  I am also considering what additional optics I want to add.  I have included screenshots and links to all the pertinent ATF letters.  I plan on carrying copies of all of them (until I get my own reply) with the firearm because there are way too many so-called experts out there who don’t know sh!t about the law, many of them being gung-ho LEO’s.  We all want to make sure we don’t get shot and stay out of Federal Pound Me In The Ass Prison.

And remember…

keep calm and carry an ar15