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Building the Most Accurate 300 Blackout AR 15

300 Blackout is a wonderfully versatile round that provides some pretty impressive ballistic coefficients, even with its large, fat, heavy bullet. Most experts agree that supersonic, 125-grain rounds fired from your typical 16” barrel garner a max effective range of 460 meters, based on hit probability.

Reaching out and touching a target with this slower-than-5.56 round means investing in an AR 15 that’ll take advantage of the supersonic loads available in 300 BLK, without sacrificing the ability to reliably (and still accurately) handle subsonic, 225-grain loads. That means configuring your black rifle with a very specific setup. Let’s dive into just what you’ll need to reach out nearly 500 meters. With our recommended setup, you’ll easily hit sub-MOA accuracy at 100 meters, too. Let’s begin!

Lower Receiver Assembly

Trigger pull and cheek weld play a huge role in the whole accuracy picture. You’ll need a good, crisp trigger that features little creep and take-up with a smooth and consistent break. Creep is the trigger’s movement before the sear disengages and the hammer falls, and take-up is that “slack” you might find in a loose, poorly machined trigger group.

We personally recommend Palmetto State Armory’s classic lower parts kit. It receives high remarks for providing a simple, clean single-stage trigger with just the round of pull weight. A USGI-like, 6-position buttstock and rifle buffer tube are included to provide a tried and proven cheek weld and sight picture.

Paired with one of our 80% lowers and jigs, this lower parts kit will afford you a well-honed, quality AR lower that features a nice hard coat anodized finish, and a crisp trigger with a consistent pull.

Upper Receiver Assembly

This one’s simple and generally won’t affect accuracy. Lucky for you, the 300 Blackout uses the same, standard upper receiver as any other AR 15. Our M4 flat top upper-equipped assemblies provide a simple but effective Picatinny rail to host your optic or scope of choice. Our full-built uppers feature chrome-lined bolt carrier groups and charging handles, so you’ll be ready to fire out of the box once you mate your upper of choice with your lower assembly.

The Critical Part: The Barrel

Picking the perfect barrel depends on whether you want to build a rifle or AR pistol (or SBR). Either way, if you want the most accurate, dead-ringer of a 300 Blackout gun, you have to stick with a quality stainless steel barrel assembly.

Of all the stainless options out there, our 416R stainless barrels provide perhaps the greatest accuracy when it comes to slinging 300 Blackout downrange. Economy stainless barrels are built from 410 stainless, while “middle of the road” barrels feature 416 base stainless. The 416R stainless alloy includes the element Molybdenum (a quality component of standard Chromoly barrels) and it features a lower sulfur content. This makes barrels built from 416R less prone to developing dangerous barrel fatigue and accuracy-killing blemishes in the rifling.

Twist Rate, Barrel Length, and Gas

Now that we’ve established 416R stainless as the barrel steel of choice, we need to figure out the other components of the equation – after all, rifling and steel alone don’t make a functional gun. For building an accurate 300 Blackout rifle, we strongly recommend sticking with a 1:8 twist, a 16” barrel, and a carbine or pistol gas system. The 1:8 twist is more optimal for those light, supersonic 300 Blackout loads, but it can still handle heavier subsonic loads.

If you’re building a 300 Blackout pistol or SBR, a 1:7 twist rate may be more beneficial with the shorter barrel. It will help stabilize those heavier, subsonic loads, which is typically what most owners shoot when building a pistol or SBR. Most community veterans agree a 10.5” barrel in 416R stainless is optimal for achieving the most accuracy in a sub-16” gun.


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