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The 300 Blackout AR-15 Build Guide (2021)

The 300 Blackout AR-15 Build Guide (2021)

Posted by on Sep 11th 2019

300 Blackout is a wickedly cool round: It can be fired suppressed or un-suppressed, subsonic or supersonic, with no modification to the weapon firing it. Whether you want to reach out and snag a deer at 150 yards, or ring steel at 50 yards (and hear nothing but that *ding*), then you’ve got to configure your black rifle or pistol the right way.

This is the intro and beginner’s guide to building an AR-15 in 300 Blackout.

We’ll look at the history of this round first. Then we’ll review parts compatibility, barrel length and twist rate, and gas systems. You’ll know everything you need to pick out the right parts. We’ll also go over ballistics data and compare performance to other common AR-15 rounds.

Let’s begin:

What is 300 Blackout?

The .300 AAC Blackout is a rifle round developed for the U.S. military’s special operations. It was approved for use in 2011. Tired of 5.56’s stopping power, top brass wanted a more powerful .30-caliber round that could match the ballistics of the Russians’ 7.62x39mm. The new round also had to provide better subsonic, suppressed performance than the 9mm being used in SMGs. It had to use existing magazines, it had to fit the current M4 platform, and it still needed a 30-round capacity.

Seeing easy opportunity, exotic ammo maker Advanced Armament Corps. (AAC) developed the .300 BLK out of their existing .300 Whisper cartridge. The Whisper cartridge was a necked down, re-sized 5.56 NATO shell casing with a .30-caliber round sitting on top. It fit in regular M4 and M16 magazines and still offered a 30-round capacity thanks to the same shell casing diameter and similar overall length to 5.56. AAC standardized the round, submitted it to SAAMI (Sporting Arms and Ammunition Manufacturer’s Institution) for testing and approval, and .300 AAC Blackout was officially born.

Building With 300 Blackout: The Parts

When it comes to picking out parts, building a .300 Blackout rifle, SBR, or pistol is almost identical to building an AR-15 chambered in 5.56 NATO or .223 Remington. In fact, many shooters convert their existing weapons because it usually only requires modifying one or two things.

AR-15 parts compatible with .300 BLK and 5.56/.223:

  • Lower receiver
  • Lower parts kit
  • Trigger assembly
  • Buffer assembly
  • Bolt carrier group
  • Upper receiver
  • Handguard
  • Magazine

These are the only parts that are exclusive to .300 BLK:

  • Barrel
  • Gas system*

Picking a Gas System

This one's easy, too: The gas system can be shared between both cartridges, but to reliably cycle .300 BLK, your weapon will need a pistol- or carbine-length gas system. A mid- or rifle-length gas system won’t provide enough energy for the bolt to cycle in the upper receiver, causing failures with feeding and ejecting rounds. Virtually all 300 BLK builds use a pistol gas system, and it's what we recommend regardless of your choice in ammo.

Reliably Cycling Subsonic Rounds

To keep things simple, here’s the rule: The slower and heavier the cartridge you shoot, the less recoil energy it produces. If you’re piecing together a sub gun and you find your bolt not cycling or locking to the rear when you shoot suppressed (or even un-suppressed), you’ve likely got an issue with your choice of gas system or buffer.

Gas Port Lengths

  • Pistol: 4”
  • Carbine: 7”
  • Mid: 9”
  • Rifle: 12”

Knowing that a pistol length gas system is nearly half as long as a carbine system, we can see how failures can be attributed to your cartridge’s expelled gasses not maintaining enough energy to cycle your bolt once they reach your upper receiver. The shorter the gas system, the more reliably your sub gun will cycle suppressed, subsonic 300 BLK loads.

Buffers and Weight

But the gas system’s only half the equation. Your buffer and spring work together, providing enough reciprocating energy to push the bolt carrier group back through the upper receiver, chambering another round and resetting the trigger and firing pin. If your bolt doesn’t lock to the rear, or if you often get failures to feed, you may be suffering from a buffer that’s just too heavy.

Heavier buffers require more energy from those blown-back gasses to fully cycle your bolt carrier. While heavy buffers are great for reducing felt recoil and bolt carrier abuse in conventional AR 15s, they do little to benefit a 300 Blackout sub gun. In this case, you may need a lighter buffer that requires less energy to cycle.

The Winning Buffer is…

In true fashion, the black rifle community has tested, argued, and gone back and forth on this topic, and the results are in: If you want to reliably cycle 300 Blackout subsonic loads with a suppressor (or simply without), you’ll want to start with a standard Carbine H buffer. This buffer weighs about 3.8 ounces and uses one tungsten and two steel weights.

If an H buffer is too heavy even still, you’ll need to swap it for a standard Carbine buffer that uses three steel weights for a total weight of just 3.0 ounces. We recommend the H buffer first, based on available data and user experiences – it helps to mitigate excessive recoil and is said to reliably cycle both loads when suppressed and un-suppressed.

Barrel Length & Twist Rate

Luckily, the barrel length you choose will work for both heavy and light 300 Blackout loads. The twist rate will matter more, but we'll get to that in a moment.

A 9" barrel is the optimal barrel length for 300 Blackout.

A nine-inch barrel will guarantee that your .300 BLK cartridges burn all their powder, achieving optimal velocity with the shortest barrel length possible. This length will provide similar performance to an AR-15 chambered in 5.56 or .223 with a 14.5" to 16" barrel. To back up the data, let's look at how the U.S. Military rates cartridges. They use a 50% hit probability to determine max effective range.

For 5.56 NATO, the military says 500 meters is the limit:

5.56 NATO (14.5″ barrel, 62-grain):

  • 100″ drop at 500 meters
  • 41″ drift at 500 meters
  • 291 lb-ft. of energy at 500 meters
  • 500 meters max effective range

For 300 Blackout, its max effective range is 440 meters:

300 Blackout (9″ barrel, 125-grain):

  • 100″ drop at 410 meters
  • 41″ drift at 470 meters
  • 291 lb-ft. of energy at 625 meters
  • 440 meters max effective range

If you've done research, you probably saw many builders opting for an even shorter barrel, around 7.5" to 8". This is still sufficient for shooting 300 BLK at close to medium range (100m to 200m). Sig Sauer even built their new 300 BLK-chambered MCX "Low Vis" carbine (which has seen service with military units) with a 6" barrel and optional suppressor. 

A 1:7 or 1:8 twist rate is the best rate for 300 Blackout.

Barrel length doesn't matter so much, because both supersonic and subsonic loads use similar amounts of gunpowder. Twist rate, however, matters a lot with .300 BLK. Supersonic loads usually weigh around 80 to 125 grains. Subsonic loads weigh at least 225 grains, sometimes more. Trying to fire two types of bullets - one weighing twice as much as the other - means picking a compromise when it comes to rifling.

Can't decide between these two rates? Ask yourself this question: Will you fire lighter supersonic rounds more often? Grab a 1:8 rate. Or, will you grab a suppressor with that juicy, $200 tax stamp and fire subsonic loads more often? If so, grab a 1:7 rate. The quicker rate does a better job at stabilizing those slow, heavy bullets and it can handle supersonic loads just as well. Reports from shooters state that even with a 1:8 twist rate, they're available to effectively stabilize most subsonic loads weighing 220 to 240 grains.

Configuring a .300 BLK Pistol

When you do build your 300 Blackout shooter, you'll probably purchase a barreled upper assembly that measures less than 16". After all, that's why 300 Blackout was made: To enjoy a shorter weapon with more firepower. But unless you go through the hoops of legally building your AR-15 as an SBR, you'll need to configure it as a pistol.

We also wrote a full guide on pistol braces and the legal stuff for 2019. This guide walks you through whether you can shoulder a pistol brace, what types of braces you can buy, and what you can and can't do with it.

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

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