Achieving superior accuracy and reliability with your 300 Blackout rifle, SBR, or pistol requires getting the right barrel assembly – it’s the single most important part of this platform, in fact. Why? Because you’re going to be handling a wide variety of grain counts and loads. If you want to use your 300 Blackout to its full effect, that also means you’ll want to be able to shoot suppressed and unsuppressed.
Shooting the same cartridge four ways is a feat not pulled off by any other rifle and load combo, and making that magic work means investing in the right barrel. Let’s break down what you should be looking for in a 300 Blackout barrel!
Barrel Length and Loads
When it comes to 300 Blackout and accuracy, velocity matters more than it does with any other cartridge. Supersonic loads are fat, subsonic loads and slow and fatter, and stabilizing them requires as much velocity as possible (while staying below the magic, supersonic number of 1,125 feet per second if shooting suppressed).
With this in mind, the optimal barrel length can really vary based on your needs and wishes. Building a long-range shooter with mostly supersonic loads? A 16” barrel will do the trick – anything longer provides diminishing returns on velocity.
Building an short-barreled rifle (SBR) with the intent of enjoying the best of both worlds? Most shooters agree that 10.5” is the magic number. This barrel length puts you in accurate SBR territory, maintaining generally 1” shot groups at 100 yards with any load – though that’s only half the equation.
Twist rate matters quite a lot with 300 Blackout, too. Because supersonic loads are usually 125 grains and subsonic loads are 225 grains, you have a ton of variance in what can be stabilized effectively. Finding the best of both worlds – a sweet spot that does both, if you will – means sticking with a 1:7 or 1:8 twist rate.
These twist rates will reliably stabilize both loads in any configuration – suppressed or unsuppressed – with good accuracy. If you’re shooting subsonic more often, stick with 1:7. The faster twist rate will stabilize these slower rounds and account for lost velocity. If you’re handling supersonic more often than not, 1:8 will do fine.
If you’re the extreme shooter who wants to push supersonic 300 Blackout loads as far as possible, 1:10 is the max you’ll want to go. This is generally only worth it if you’re sticking with ultra-light, 110-grain loads with the intention of shooting beyond 500 meters.
This is another unique factor that plays a bigger role in 300 Blackout-chambered AR 15s. Barrel profiles usually make marginal (ever hardly ever measurable) differences in other platforms, but when shooting a big .30 caliber round through a relatively thin AR 15-type barrel, it matters.
To keep it simple, we recommend sticking with a heavy profile.
This thicker, more rigid barrel will afford the most accuracy and performance possible. If you’re shooting suppressed, you’ll be generating a lot of heat – that extra steel will help to mitigate heat soak and improve long-term performance.
While not necessarily uniquely important when shooting 300 Blackout, getting quality barrel steel will always mean enjoying longer barrel life and better accuracy. Again, because stabilizing and accurizing 300 Blackout is more of a chore than with other cartridges, every factor counts.
Because of that, we recommend sticking with the top-tier stuff: 4150 Chromoly, or 416R stainless.
These more rigid, more machinable steel alloys produce the best barrels and they each suffer from fewer non-metallic inclusions that kill accuracy and barrel life. A heat-treated barrel with a Mohs hardness rating of RC 28 to 32 will be the best choice for barrel life and accuracy.
Heat treatment produces hardened rifling that can stand up to the wide variances of shooting different 300 Blackout loads, and it negates the need for chrome lining – another costly process that can also kill accuracy if done poorly.
Gas System Length
This one’s simple: Stick with a pistol- or carbine-length gas port. Shorter is better in this case. The lesser the distance your gas has to travel, the more reliably it’ll cycle your bolt carrier group when shooting either supersonic or subsonic loads, suppressed or unsuppressed. Again, the goal here is to invest in a 300 Blackout barrel that can manage all four methods of shooting.