Changing From 5.56 NATO To .300 Blackout

Aaron Spuler is a firearms enthusiast and recreational shooter. Follow more or his work at The Weapon Blog

If you’re up on recent developments in the AR community, you’ve likely heard of the new, interesting round, the .300 blackout. This new ammunition, its proponents will tell you, gives your AR some additional capabilities that make it well worth the money and effort to switch over from standard 5.56mm NATO. The great thing is you can do it yourself with parts that you can order online.

Why Switch to .300 Blackout?

In this piece, we’ll explain why people choose to switch an AR to fire the .300 blackout round. From there, we’ll cover what you need to do it yourself in terms of parts and walk you through the process. Finally, we’ll make some recommendations as far as getting the most out of your new .300 blackout upper receiver setup.

The .300 blackout is a relatively new cartridge that responds to some of the main critiques of the 5.56mm, namely that it lacks stopping power and is difficult to suppress. The former is part of a longer debate: the US used a .30 caliber round in the m14 but switched to a smaller, 5.56mm cartridge for the M16. This was done in the 1960s, and even as recently as the Global War on Terror, some in the military have been trying to get the 7.62NATO back into the mix with the M14, FN SCAR, and the AR10.

While the .300 blackout lacks the longer-range capability of the 5.56mm, the heavier bullet means that, under 300 yards or so, the larger round hits harder and, vitally, moves much slower. The slower .300 blackout can be easily loaded as a subsonic round, meaning it is excellent for use with suppressors. So, if you want something that walks softly and carries a big stick, then the .300 blackout is well worth a look.

All You Need Is a New AR-15 Upper Receiver

There are two ways to get set up for a .300 blackout. Of course, that’s assuming you already own an AR15. If that’s the case, your existing lower receiver and magazines will work as-is which presents you with substantial cost savings over buying a whole new rifle (or pistol!).

If you’re comfortable with a barrel swap, then all you technically need is a .300 blackout barrel. This is the cheapest way to go but would leave you needing to change back to a 5.56 barrel if you want to shoot that cartridge.

Otherwise, the easiest way to go is to simply buy a .300 blackout upper receiver: this means all you’d need to do to switch calibers is to change out magazines with correct ammo, and pop on the relevant upper receiver.

Assuming you decide to go with the barrel swap route, it’s highly recommended to watch some videos on it first. From there, you’ll need to strip the furniture off your current upper, unscrew the gas block and remove the gas system, then use an armorer’s wrench to get the barrel off.

To get the new, .300 blackout barrel on, simply reverse the steps. Again, there are excellent video resources on this.

Putting a complete upper on is much, much easier: pop the front and rear receiver pins on the old one, place it in a safe spot for storage and pop the new one on. We expect a lot of people will go this route.

.300 Blackout is Great to Suppress

To wrap up, we have two recommendations for people switching to .300 blackout. The first is to also seriously consider a suppressor: it’s one of the better uses for a .300 blackout, and having the capability to shoot subsonic, quiet ammunition is great.

Second, it’s important to keep your ammunition organized. 300 blackout and 5.56mm fit in the same magazines, and, ideally, should not chamber in the incorrect barrel. Accidentally running 300 BLK ammo in a 5.56 barrel will catastrophically fail. But to be on the safe side, it makes sense to keep the magazines separate. Assuming you use standard MAGPUL mags, you have two options.

First, you can get different colors of PMAG baseplates. Also, there’s the old standby of using markers. Either way, a simple, color-coded way to tell magazines apart is the way to go.

Conclusion

As you can see, it’s fairly easy to switch from 5.56mm to .300 blackout. With just a new upper receiver and a few new magazines, you’ll have what feels like a whole new rifle with some excellent new capabilities.