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How to Reliably Cycle Subsonic 300 Blackout in Your AR 15

300 Blackout can already make for a finicky, picky gun – shooting suppressed with subsonic loads and enjoying 300 Blackout the way it was meant to be enjoyed can exacerbate those problems to the point of hair-pulling frustration. Let’s clear the smoke before that happens and break down just what you need to build a reliable, subsonic 300 BLK shooter that’s ready for a can.

Pistol Length is What You Need

To keep things simple, here’s the rule: The slower and fatter the cartridge you shoot, the less gas and energy it produces. We’re not talking about “stopping power”, we’re talking about the very important event this energy also performs: Cycling your weapon. If you’ve pieced together a 300 Blackout sub gun and you find your bolt not cycling or locking to the rear when you shoot suppressed (or even unsuppressed), you’ve likely got an issue with your choice of gas system or buffer.

Gas Port Lengths

  •         Pistol: 4”
  •         Carbine: 7”
  •         Mid: 9”
  •         Rifle: 12”

Knowing that a pistol length gas system is nearly half as long as a carbine system, we can see how gas system lengths can be attributed to your cartridge’s expelled gasses not maintaining enough energy to cycle your bolt once they reach your upper receiver. The shorter the gas system, the more reliably your sub gun will cycle suppressed, subsonic 300 Blackout.

Buffers and Weight

But the gas system’s only half the equation. Your buffer and spring work together, providing enough reciprocating energy to push the bolt carrier group back through the upper receiver, chambering another round and resetting the trigger and firing pin. If your bolt doesn’t lock to the rear, or if you often get failures to feed, you may be suffering from a buffer that’s just too heavy.

Heavier buffers require more energy from those blown back gasses to fully cycle your bolt carrier. While heavy buffers are great for reducing felt recoil and bolt carrier abuse in conventional AR 15s, they do little to benefit a 300 Blackout sub gun. In this case, you need a lighter buffer that requires less energy to cycle.

The Winning Buffer is…

In true fashion, the black rifle community has tested, argued, and gone back and forth on this topic, and the results are in: If you want to reliably cycle 300 Blackout subsonic loads with a suppressor (or simply without), you’ll want to start with a standard Carbine H buffer. This buffer weighs about 3.8 ounces and uses one tungsten and two steel weights.

If an H buffer is too heavy even still, you’ll need to swap it for a standard Carbine buffer that uses three steel weights for a total weight of just 3.0 ounces. We recommend the H buffer first, based on available data and user experiences – it helps to mitigate excessive recoil and is said to reliably cycle both loads when suppressed and unsuppressed.

Need a Little Help?

The above configuration (pistol gas system with a Carbine H buffer) should be sufficient for cycling subsonic 300 Blackout loads in any gun. If you’re still running into issues, like an occasional failure to feed or failure to lock the bolt to the rear, an upgraded and lighter buffer spring and bolt carrier may help. Some companies have produced lightened recoil springs and lightened bolt carriers that are specifically designed for subsonic 300 Blackout.

With these two (or possibly four) pieces, you’ll have an AR 15 that can reliably cycle and shoot subsonic 300 Blackout loads day and night, suppressed or unsuppressed!


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