With all the calibers available to 80 percent AR-15 builders out there, 300 Blackout is probably one of the more popular options after .223 Remington or 5.56 NATO. Today, we’ll be going over why the 300 BLK gets so much attention, its benefits and limitations.  

We’ll start by breaking down the caliber’s name .300 AAC Blackout. .300 is the caliber and you can see in the photos above and below how much more of a stout round it is compared to .223 Remington or 5.56 NATO. The AAC is often left out of the name but it’s actually the name of the company which designed this round — Advanced Armament Company. 

The .300 BLK’s predecessor comes from the .300 Whisper which was specifically designed to be shot suppressed. But, the Whisper and the Blackout are not the same. Their differences are similar in the way that the .223 and 5.56 differ in their neck diameter of the cartridge. For .300- rounds, it is safest to assume that the Blackout and Whisper are not interchangeable though with the right ammunition it is possible to fire a .300 Whisper out of a .300 BLK rifle. 

From left to right: 55-gr. .224, 77-gr. .224, 125-gr. 300 BLK and a 245-gr. lead subsonic 300 BLK bullet
From left to right: 55gr. .223, 77gr. .223, 125gr. 300 BLK and a 245gr. lead subsonic 300 BLK projectile.


Let’s get right into the benefits of what makes the .300 BLK awesome. This specific caliber has great terminal ballistic data. What does that mean? Well it’s also referred to as wound ballistics, which is an offshoot field of study of how projectiles behave when they hit and transfer its energy to a target (aka penetration). Based on the design of a bullet and its speed of impact, we can determine how effective it will be upon impact with a target. Since the .300 BLK is a slightly larger round it’s always going to make a bigger hole in a body than 5.56 will in CQB. So if that’s what you’re looking for, you’ve come to the right place. 

Speaking of CQB (close quarter battle), part of what makes the .300 BLK so optimal is its ability to fully burn powder from a round being fired out of a 9 inch barrel. Comparatively, .223 and 5.56 need almost double that length to do the same (if not more). The benefit that this provides is the ability to take advantage of a much shorter weapon which will reliably hit any target under 300 yards. 

The Blackout is able to cycle both subsonic and supersonic rounds without any modifications. Paired with subsonic rounds and a suppressor, this combination renders the recoil on a .300 BLK rifle almost to nothing with significantly less sound. That may be why more SWAT and SRT units across the country have begun to use this caliber in their rifles. It’s a practical conversion for civilians too because all you need to do is swap out the barrel. Though, if you’re like us it’s really just a good excuse to build out a new complete upper. 

If you reload your own ammo it’s also easy to start doing so for .300 BLK because all you need are new dies to accommodate the cartridge’s size. The process of prepping the brass for .300 BLK is the same as .223 and 5.56. Perhaps the best part of reloading .300 is that it uses less powder than .223 does which provides some savings in that aspect. In case you’re short on brass, you can actually take 5.56 casings and trim it down to the correct size for .300 BLK as a sort of last resort measure because it can be a pain to do that.

In terms of application, the .300 BLK is a fantastic choice in hunting larger game and there is a wide range in projectile choices. Granted, a lot of hunting is just about shot placement and you can hunt just about anything with 5.56; .300 BLK might come in handy if you’re trying to wipe out a horde of feral hogs. As we mentioned briefly above, part of the huge appeal of .300 BLK is their ability to utilize short barrel lengths. If your hunt is going to be long and involves a lot of trekking, add a LAW Folder and you could literally have a backpack gun less than 14″ while folded.


Honestly, there isn’t a whole lot of cons here but these are some factors you may want to consider: 

Bullet Drop – If you’re looking to shoot long distances, 5.56 is going to be the better option as it shoots a lot flatter because it flies at higher velocities. It’s not to say that you can’t hit targets at distances further than 300 yards but with .300 BLK there is a massive bullet drop rate of 12” once you start aiming at targets at the 300 yard mark which can cause very spotty accuracy especially if you factor in weather. 

Price – Though this wasn’t always the case, .300 BLK is typically twice the price of 5.56 these days making it a very pricey round given that we’re currently still dealing with COVID pricing. If you already have a reloading set up, it might offset some of that cost provided you can acquire all the necessary components.

m855 ballistics
m80 ballistics

Home Defense – While the .300 BLK is great for CQB, it might not be the best option for home defense because of the round’s penetration ability. Over-penetration is a serious factor that people should consider if they live in small home or apartments. If you miss your target, there’s a good chance that bullet will pass through multiple walls before fully stopping. Whereas, 5.56 bullets break apart more easily and have less tendency to behave like so. See Jerry Miculek shred some ballistics gel with various .300 BLK rounds here.

If you’re looking to build yourself a new 80 percent 300 blackout rifle, check out our complete upper kits here!