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How to Load 300 Blackout with 5.56 Brass

As the name implies, 300 Blackout is a .30-caliber cartridge. In the land of common AR 15s, .223 and 5.56 are .22-caliber cartridges. But fret not! If you’re on the fence about buying a black rifle in a different caliber, thinking you’ll have to deal with buying different brass for reloading and shooting, you’re wrong.

300 Blackout shares many characteristics and weapon parts with 5.56 or .223 AR 15s – they share the same lowers, the same trigger assemblies, the same bolt carrier groups, the same uppers, and there’s good reason for that: You can actually turn your average, run-of-the-mill 5.56 brass into 300 Blackout brass that’s perfectly suitable for supersonic and subsonic loads, and the process is rather simple:

Getting the Right Case Thickness

To start, you’ll simply need to neck down 5.56 brass. Before we get any further, though, it’s important that you’re using the right stuff: You’ll need brass with a case thickness of 0.01 to 0.0115”. The maximum case wall thickness is 0.012” – anything thicker, and you’ll run into fitment issues. Anything thinner, and your cases may not handle the added pressure and weight of 300 Blackout.

Cutting to The Right Case Height

Lucky for you, the most common 5.56 makers like Remington, Lake City, and Federal all provide just the right amount of case thickness. Once you’ve got the right brass, you’ll need to trim the cases. That means cutting your cases down to 0.400” on the nose. Where necking down other calibers and cartridges usually means only a little bit of trimming, this is like performing an amputation, relatively speaking. We recommend using a cutoff saw and then cleaning up your cuts with a conventional trimmer.

Annealing for Performance

Next, you’ll need to anneal the case necks. If you’ve ever wondered why your average .223 or 5.56 cases sport a discolored, blackened hue near where the bullet is seated, well that’s annealing. Annealing means you’re heating up the tip of the brass to soften it. This makes neck tension more consistent and it provides a better seal in the chamber when you pull the trigger. You don’t have to anneal your cartridges, but with this much cutting and with accuracy a critical piece of the fun factor, we strongly recommend it.

Annealing is achieved by exposing the brass neck to 800 degrees for about 6 seconds. The process doesn’t take long, and at-home annealing torches are readily available online.

Sizing the Cases

This part’s simple: You’ll run your cut (and hopefully annealed) cases through a 300 Blackout sizing die. This will pinch the cartridges to the appropriate neck angles, and once pressed, you’re ready for powder and bullets. There are no special steps needed otherwise! If this surprises you, just remember – 300 Blackout was originally developed in J.D. Jones’ own garage using this very same process.

Convenience and Accuracy

So there you have it, you can take common 5.56 cartridges, cut ‘em, heat treat ‘em, and press ‘em and you’ll wind up with perhaps the easiest and least expensive way to hand load 300 Blackout for even better accuracy. Few other cartridges exist on the market that offer this kind of convenience, and no other rifle exists with this kind of unique interchangeability. It’s just another reason for you, the average AR 15 owner who enjoys reloading, to love 300 Blackout.


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