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How to Build a 300 Blackout SBR

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The 300 Blackout has been a staple of the special warfare community thanks largely to its respectable ballistics, accuracy, and its dual function as a supersonic and subsonic round. In fact, in March US SOCOM released an official request for manufacturers to develop lightweight, 300 Blackout-chambered SBRs to replace their existing M4s and Mk. 18s. So if a 300 Blackout SBR’s good enough for SOCOM, it must be something you want, right? But why?

Why You Want a AR 15 SBR in 300 Blackout

For starters, 300 Blackout offers substantial accuracy in short barrels. Many in the community report achieving 2” shot groups at 100 yards with factory cut brass in a 10” barrel. Light weight is a huge advantage, too: That request from SOCOM included that the new 300 Blackout SBR weigh no more than 5.5 lbs. While that sounds like snake oil in the AR 15 world, achieving a featherweight rifle like that is easy with such short barrels and the accompanying furniture. Lastly, SBRs are just cool. They’re compact, easy to maneuver with, ergonomic, and surprisingly affordable to build.

So, your question has been answered: Owning a 300 Blackout AR 15 SBR is as awesome as it sounds. Now it’s time to deconstruct how to build (and legally own) one. Legally owning an SBR means submitting some stuff to the ATF – specifically, a little piece of paper called ATF Form 1. This is a form you’ll use to register a firearm, and it’s easy to fill out. Once you fill it out, you’ll send a copy to the ATF and your local sheriff. You’ll also pay $200 and wait to receive what’s commonly called a “Tax Stamp”. This is like a certificate saying it’s legal to an SBR.

While you play the waiting game for your Tax Stamp, you can still build your SBR to the point of near completion! You can even enjoy it as a functional AR 15 pistol until it’s ready to receive a buttstock, but we’ll get to that. First, let’s start with the meat of the thing, the upper.

Start With an AR 15 Pistol Upper

Lucky for you, an AR 15 pistol upper is not considered a firearm, nor is any other part of your future 300 Blackout SBR, save for the lower receiver itself. That means you can part together your future SBR by starting with a full-built 300 Blackout Pistol Upper and even finishing your weapon as an AR pistol until you get your Tax Stamp.

Because 300 Blackout is essentially two rounds in one (125-grain supersonic and 225-gain subsonic), you’ll want a barrel and gas system that can reliably cycle and accurately shoot both. For that, we recommend a 7.5” barrel for the best balance of size and accuracy, though a 10” barrel works well, too. A 1:7 or 1:8 twist rate works best for handling both loads. We also recommend sticking to a pistol or carbine gas system – anything longer will likely result in poor cycling.

Our 7.5” AR 15 Complete Pistol Uppers provide all of these features with 4150 Chromoly steel barrels and M4 flat top upper receivers. Each upper’s muzzle is outfitted with a 5/8 x 24 thread pitch for easy suppression. Each upper we sell also includes a chrome-plated bolt carrier group and charging handle. Next, you’ll need a good lower and you can go about this two ways:

Option 1: 80% AR Lower

This is the most affordable (and in our opinion, the most fun) way to build a 300 Blackout SBR, or really any AR 15 Pistol. All you need is a pistol lower parts kit, an 80 percent lower, an 80 lower jig, and a router. We sell the full kit you need to go from parts to finished pistol, and it ships straight to your front door. You’ll need a pistol buffer tube to build your temporary 300 Blackout AR 15 pistol. Once you receive your Tax Stamp, you can swap out your tube and buffer for a buttstock, and you’re set!

Keep a few things in mind if you decide to build an 80% lower for your SBR. You’ll need to engrave and serialize your lower first. It’ll need the following info:

  •         Name of gun maker (that’s you!)
  •         Location (City, state where you’re building)
  •         Caliber
  •         Serial Number (001 works for your first NFA item, or 666 if you’re feeling badass)
  •         Model Number (If you include a model number on your Form 1 it must also be engraved)

Option 2: Stripped AR Lower

This option is a little less exciting and rewarding, though admittedly it’s simpler, too. You’ll need a stripped AR 15 lower receiver and some of that other stuff we mentioned earlier: A lower parts kit and pistol buffer assembly.

Option 3: Full-built RA Pistol Lower

This is the simplest (though least rewarding) method to owning a 300 Blackout SBR. You’ll simply buy a full-built AR 15 pistol lower and slap your 300 Blackout Pistol Upper on it. Once you receive your Tax Stamp, you can swap out your buffer and tube for that buttstock to complete your SBR.

In the Meantime, Use an Arm Brace

The ATF surprisingly ruled that it’s legal to shoulder an AR 15 pistol’s arm brace, and that’s good news for you! With our KAK Shockwave Arm Brace, you can essentially have an AR 15 pistol that looks and functions as close to an SBR as possible, without the Tax Stamp.

We hope this guide helped you on your journey to owning an AR 15 SBR. If you have more questions about what upper or parts will work best for your 300 Blackout build, just call us or use our chat function to talk to an expert!


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