Building a 300 Blackout-chambered AR 15 pistol is a sort of art. You have to balance that compact, aesthetic quality with the ability to stabilize your pistol and accurately shoot both subsonic and supersonic loads with an acceptable level of accuracy.
For a dual-purpose round whose grain count can vary by over 100 grains, and whose velocity can range from 2,300 fps down to just 1,000 fps, this is no easy task – especially for a pistol-length AR upper. So, let’s let you in on the secrets you need to know, so you can build a badass, accurate, reliable AR 15 pistol chambered in 300 Blackout.
To keep things simple regarding our 300 Blackout Barrel options, you don’t want to go any shorter than a 7.5” barrel. Other makers claim they can manage stability and some measure of accuracy below 7”, but you’ll be struggling to hit any respectable distance consistently. The most optimal sub-16” barrel length that offers the best balance of compactness and accuracy is 10.5”. Many users report MOA accuracy at 100 meters using a 10.5” barrel, though some folks who’ve hand-loaded their cartridges have said they manage the same with 7.5”.
You’ll want to stick with a 1:7 twist rate, and generally, 1:8 is high as you can go with a pistol build. You’ll be working with a much shorter barrel, and that means you’ll have less distance to stabilize your shots before they leave that muzzle. A faster twist rate like 1:7 will put enough spin on either a supersonic or subsonic load, affording good shot groups.
Because we’re dealing with shorter barrels on pistol builds, you’ll need something that’s higher quality than your average rifle barrel – again, you have less real estate to make every shot count. You’ll want a more rigid barrel that suffers less flex, and you’ll want harder, sharper rifling.
That generally means sticking with either a 4150 Chromoly or 416R stainless barrel. These two barrel alloys are the hardest and more pure options available, and though they might cost a few dollars more, they’re worth the added accuracy and reliability.
Recoil, Buffers, Springs, and Reliability
Once you start slamming rifle rounds through short barrels, you can begin to see reliability issues crop up. We’re talking about bolt carrier groups not locking to the rear, not cycling fully, failures to feed, failures to eject, you name it. Heavier buffers might suffice for rifles, but when you build an AR pistol, less energy is being transferred to the buffer and spring.
That energy loss occurs because you’re not shouldering your pistol, so more recoil energy is being taken up by the movement of the entire weapon and the movement of your arms absorbing that energy. This is where you can experience those failures, especially when shooting suppressed, subsonic 300 Blackout.
To stop these malfunctions from happening, you’ll need a specific buffer setup. For a pistol, that means sticking with a Carbine buffer and spring. At around 3.0 ounces, Carbine buffers weigh the least.
Slapping iron sights on your AR 15 pistol is acceptable, but you won’t be as accurate as you’d like. This is because even in a pistol configuration, AR 15 front and rear sights are spaced pretty far apart – and you’ll experience more error with your sight picture.
To get on target and maintain tight groups, we strongly recommend investing in a simple red dot. These single-plane sights are free of the quirks and problems you’ll encounter using just irons. They provide quick target acquisition and they’re fun to use. The Trijicon MRO, Vortex Sparc II, and EOTech Transverse are all great places to start looking.
Stability and Comfort
We strongly recommend investing in a stabilizing brace like the KAK Shockwave or the CAA Saddle. If you want to keep things simple and affordable, at least get a buffer tube cover. These cloth covers will help prevent rubbing, bruising and blistering when you AR pistol inevitably rides against your forearm.
We also recommend investing in a foregrip. The Magpul Angled Foregrip does wonders for mitigating the effects of recoil and barrel rise by allowing you to push into your pistol, against the energy of the recoil itself. You can check it out here!
All Other Things Considered
When you build an AR pistol chambered in 300 Blackout, you get to enjoy the convenience of using all standard AR lower receiver parts – including the stripped lower and parts kits themselves! With this mind, and armed with those valuable secrets above, it’s time to get building.