Everyone and their best friend has an AR chambered in 5.56 or .223, whether it’s a full-size rifle or an ARP.
Now, here’s the thing. These chamberings make sense with a full size rifle because the longer barrel extracts the desired performance from both of these cartridges.
Once you step down in size to an ARP build, that dynamic shifts. There are other, better chamberings for an ARP, or AR pistol.
Notably, the .300 Blackout, which offers better stopping power and performance from shorter barrels. Here’s what you should know before you buy that .300 Blackout AR upper.
Your Next .300 Blackout AR Upper Should Be for an ARP: Here’s Why
There are things that 5.56 does better than .300 Blackout, but at the same time, many of these advantages are relative and evaporate once you restrict the scope of the inquiry to AR pistols.
First and foremost, the .300 Blackout offers better stopping power. If you are building or intend to keep this ARP for defensive applications, such as home defense or as a truck gun, the .300 BLK objectively wins out.
The .300 Blackout is loaded with larger, heavier bullets that create larger permanent wound channels than 5.56. There’s no way around this.
On top of that, the .300 Blackout cartridge is often loaded with very heavy bullets, sometimes in the area of four times the mass of 5.56 bullets, that are designed to offer subsonic performance.
The deal here is that you get similar if not greater stopping power, but since the bullets are subsonic, you won’t blow your eardrums out firing in a closed room or hallway. So you get a cartridge that’s safer for your ears and produces more stopping power than 5.56 for neutralizing threats. That’s no small matter.
If you want to highlight this advantage even more sharply, you could add a suppressor to the build and make the platform even that much more quieter. Of course, this will add weight and length to the build, which will counteract the next fact. But it should still be considered.
There is also the fact, and this is a more important consideration, that on top of all this, .300 BLK performs much better from a shorter barrel than 5.56 does.
To get the desired ballistic performance from 5.56, you really need a barrel at least 16 inches. Perhaps there is some grace on either end of that but you really need a longer barrel. The 5.56 is a hot, fast, flat-shooting cartridge and you need that barrel length to ensure full propellant charge deflagration along with bullet acceleration.
This is not the case with .300 Blackout, which by comparison is a slower rifle cartridge. It will perform much better from shorter rifle barrels as short as 10 inches, and in some cases, even less. In fact, you can get complete powder charge deflagration from a barrel this short or even short.
This is the other big reason, and perhaps the most important one, that a .300 Blackout AR upper, specifically a shorter upper intended for an ARP, is a better move than a short 5.56 AR upper.
You will lose performance with a shorter barrel if you build the ARP around 5.56 but you will get exactly what the .300 BLK offers from a shorter barrel, with no compromises.
So, at the end of the day, you can build a more compact, harder-hitting ARP as long as you choose a more suitable cartridge, like .300 BLK for the build instead of the standard 5.56.
Why Not 5.56?
Now, to recap, there are a few reasons that, ultimately, while 5.56 is a good chambering for a carbine or a rifle (or even an SBR) it is not ideal for an AR pistol. Here are some of the big ones:
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5.56 does not perform well in shorter barrels.
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5.56 does not hit as hard as .300 BLK.
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While 5.56 shoots flatter, that’s no advantage at the close ranges in which defensive platforms are implicated.
At the same time, it is important to note that .300 BLK will be harder to find, will be more expensive, and will produce slightly more recoil than 5.56. But if you can live with those things, it’s a better chambering for an AR pistol.
Here for a .300 Blackout AR Upper?
We carry short uppers for AR pistol builds, including micro ARP builds, along with complete .300 Blackout AR uppers. Check out our full collection and if you have any questions about recommended barrel length, buffer weight, or anything else related to compatibility or performance, get in touch with us at Sales@MCSGearup.com and we will assist.