How to Build The (Best) 300 Blackout AR-15

This writer's built numerous 300 BLK-chambered AR-15s. After testing thousands of rounds with various barrels, gas systems, buffers, and even magazines, I can confidently declare there is one, single, overall best configuration. Here's how to build the best 300 Blackout AR-15.

How to Build a 300 BLK AR-15: The Parts

If you're reading this guide, you probably already know enough about the AR-15 to avoid a total beginner's introduction. We're going to focus on simply picking out the optimal parts for this build -- but if you want to start at square one, we do, of course, have an in-depth beginner's guide: How to Build an AR-15 (The Complete Parts Guide).

("What is 'the best' setup?")

When it comes to 300 BLK, "best" means building a rifle that does what this dual-purpose, .30-caliber cartridge was meant to do: Chamber and reliably fire both supersonic and subsonic ammo, with or without a suppressor, without having to change parts, while enjoying good accuracy with optimal muzzle velocities. With that goal in mind, let's pick the optimal parts.

The 300 Blackout Lower

The ideal 300 Blackout lower is nothing more than a mil-spec, forged, 5.56/.223-type lower receiver with a standard lower parts kit, and a mil-spec buffer assembly. The 300 Blackout AR-15 does, in fact, use all of the 5.56-chambered AR's lower components.

You could simply throw a 300 Blackout upper onto an existing 5.56/.223 lower receiver, and it'll run perfectly fine. Well, mostly. If you want to build a truly "perfect" 300 BLK AR-15, there are some differences in the buffer and magazine that need to be considered.

What buffer weight for 300 BLK?

You'll see a lot of recommendations across the internet, here. Many say you should use an H2 buffer for 300 Blackout. Those folks are wrong. After testing four different 300 Blackout barrels -- each with different gas port diameters -- I found you need an "H" buffer to reliably cycle subsonics without a suppressor.

Remember, the best 300 Blackout AR-15 should be able to fire subsonics with or without a suppressor. Without a suppressor, using an H2 (or heavier) buffer can cause the bolt to short-stroke, which results in a failure to cycle and potential jams. 

Some barrel makers drill smaller gas port diameters than others, which results in less gas making it to the bolt carrier group. So, to be on the safe side, stick with an "H" buffer. Even a carbine buffer will work just fine, if you're merely converting an existing "mil-spec" 5.56 lower for this cartridge. You'll just notice more felt recoil.

Can you use 5.56/.223 magazines for 300 BLK?

Technically, yes, but you shouldn't. In this writer's (extensive) experience, 300 Blackout rounds do, in fact, cause failures to feed and double feeds when loaded up to the 30-round capacity of a 5.56/.223 magazine.

It's recommended you invest in 300 Blackout-specific AR magazines. These magazines have a shorter pusher with a steeper angle atop the follower, which alleviates these feeding issues. Besides this minor change, 300 BLK magazines are identical to 5.56/.223 magazines, so they'll fit in your standard lower receiver without any modifications necessary.

What lower parts kit for 300 BLK?

A standard 5.56/.223 lower parts kit is truly all you need. If you're building your 300 Blackout AR from scratch, follow our instructions for putting your lower receiver assembly together here: How to Install The AR-15 Lower Parts Kit.

The 300 Blackout Upper

The 300 Blackout upper is, like the lower, makes use of primarily 5.56/.223 components. You'll use a regular ole mil-spec stripped upper, a standard bolt carrier group and charging handle, and a standard AR-15 handguard. But like the lower, there are some key differences when it comes to the gas block, gas tube, and barrel.

The Best Barrel Length

Although 300 Blackout can gain more velocity with a barrel length up to 16", the extra velocity may not be worth the extra length and weight. 300 Blackout was specifically designed for short-barreled rifles, which means a short barrel is best.

The optimal barrel length for 300 Blackout is 9 to 10.5 inches.

The 9" barrel is the minimum length required to effectively stabilize the heaviest subsonic loads with the recommended twist rate (more on that below). 

Any barrel length beyond 10.5" adds negligible amounts of extra velocity to both supersonic and subsonic loads. In other words, going with a longer barrel means adding far more weight to your rifle while receiving little extra accuracy and range.

The Optimal Twist Rate

Firing light and heavy bullets without swapping barrels means picking a twist rate that can handle both rounds. The 1:7 twist rate is best for 300 BLK. Although 1:8 works in most cases, it may not effectively stabilize subsonic loads in very short (< 7") barrels, and ultra-fast twist rates that have come onto the market in recent years, like 1:5, are designed exclusively for those ultra-short barrel.

The Gas System

Subsonic 300 BLK loads produce far less gas than their supersonic counterparts. To make up for this lack of energy, the shortest possible gas system must be used. You'll need a pistol-length gas tube for your 300 Blackout build.

Converting Your Existing Upper?

We've got instructions for that, too! Just follow our guide: How to Build an AR-15 Upper. References the later steps in the guide to remove your existing handguard, gas system, and barrel, then install your new 300 BLK parts. If you've already got a free-float handguard, you can reuse your handguard and barrel nut!

We also carry barreled 300 BLK uppers! Make converting your existing AR-15 even easier by just swapping the upper itself.

(Legally Building an SBR)

If you install a stock on your 300 BLK AR-15 -- and if you followed our recommendations above and chose a barrel less than 16" in length -- your AR will be considered a short-barreled rifle, or SBR. Owning an SBR without registering it is illegal. 

In the meantime, don't install a stock on your AR (keep it configured as an AR pistol with a naked buffer tube), and do follow our guide on how to register an SBR before installing a buttstock.