The AR world is absolutely massive, and you can chamber an AR in everything from .22 LR to .45-70 Government. The AR is often adapted to function in calibers it was never designed for shooting.

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While that’s cool when it happens, what’s even cooler is when rounds are designed specifically for the AR platform.

One such round is the .300 Blackout. The .300 BLK has easily been the most successful new caliber in the last 20 years.

Read on to see how to get your standard 5.56 AR-15 ready to use .300 BLK.

Summary of Our Top Picks

1. 

   Most Affordable

   PSA .300 Blackout Uppers

   PSA does it again with sub-$200 uppers that work.

   $199 at Palmetto State Armory
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2. 

   Editor’s Pick

   Ballistic Advantage .300 BLK Uppers

   The 10.3″ BA upper is the basis of my truck gun build.

   $504 at Ballistic Advantage
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3. 

   Runner-Up

   Aero Precision M4E1 Assembled Upper Receiver .300 BLK

   Another proven company for complete uppers.

   $475 at Aero Precision
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4. 

   Another Affordable Option

   Radical Firearms .300 AAC Blackout Upper Assembly

   Another affordable option if you have your own BCG or CH.

   $225 at OpticsPlanet
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5. 

   Short Stroke Piston Option

   Brownells BRN-180 Uppers

   Go the folded brace route with a BRN-180 upper.

   $900 at Brownells
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Why the .300 Blackout Succeeded

The .300 BLK became a behemoth in the industry because of its direct compatibility with existing AR-15 rifles since it is compatible with almost every standard 5.56/223 AR-15 part.

This includes magazines, bolt carrier groups, upper and lower receivers, rails, buffers, buffer assemblies, stocks, etc. Really, the biggest change is the barrel and the gas system.

Other than that, it’s a standard AR-15.

This makes it possible to convert your 5.56 AR-15 into a .300 BLK AR-15 with nothing more than a swap of the upper receiver. 

That’s it. 

You can use the same mags, same BCGs, etc.

Why You May Want a .300 Blackout Upper

Besides the easy conversion between 5.56 and .300 BLK, the .300 BLK does a lot the 5.56 can’t.

First off, it’s a very versatile load with projectiles weights ranging from 110 grains to 220 grains. The 110-grain rounds are supersonic, and the 220 subsonic.

A subsonic round is much easier to suppress and much quieter when suppressed. The .300 BLK also reaches peak ballistic potential from a 9-inch barrel.

The 5.56 requires a 20-inch barrel to reach peak ballistic performance.

I won’t dive too deep into the 5.56 vs. .300 Blackout debate because we have an article on that.

In my opinion, the two rounds are designed for different tasks, and having both simply opens up your options.

What I will do is point you to a few of the best .300 BLK upper receivers so that you can make your transition to this wonder-round.

Best .300 Blackout Barrel Length?

This question comes up a lot…and the answer varies depending on your end use and if you’re using a suppressor.

We asked Clint Hanson, the founder of Ballistic Advantage, and his go-to barrel length is 10. inches for reliability.

The 10.3-inch length is “not too short and not too long from an all-around/fit every human size standpoint” because you have to figure in the handguard and potential suppressor length.

Once you go under 8 inches or so, you can run into cycling issues since the .300 BLK is a low-pressure round.

So if compactness is the name of the game…make sure you have a suppressor to add some backpressure and also enable you to run a longer handguard.

“Every length has a problem it’s solving, but the 10.3 inch is the go-to and most versatile.”

Best .300 Blackout Uppers

1. Palmetto State Armory

PSA makes a huge range of items for the AR-15 platform and, for that matter, the AR-10 also.

One of the many things they cover is the .300 BLK in uppers of all sizes, from 7.5-inch barrels all the way to 16-inch barrels, with every standard size in between.