5.56 vs. 300 BLK: Which Do You Need?

By Tom McHale
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5.56 vs. 300 BLK: Which Do You Need?

February 13th, 2020

5 minute read

Ford or Chevy? 9mm vs. .45 ACP? 5.56mm NATO vs. 300 Blackout (BLK)? The debates never end. While I can speak to the case for the vehicle brand wars, the calibers disagreements often persist because there’s usually a nugget of truth in the arguments for either side. Then there’s the issue of context. These disagreements are often battled in general terms of what’s “best” with no defining parameters for how “best” is determined. Is a Bugatti Veyron better than a Jeep Wrangler? Not if your purpose is to drive up a mountain on a pack mule trail.

Springfield Armory SAINT Victor Pistol 300 BLK (left) and the SAINT Edge Pistol (right) in 5.56mm.

Let’s take a closer look at the 5.56mm NATO vs. 300 BLK question. For a “real life” comparative illustration, we’ll consider two very similar AR pistols that vary primarily by caliber: the Springfield Armory SAINT Edge Pistol (5.56mm) and the SAINT Victor Pistol chambered in 300 BLK.

Why the BLK?

The goals of the 300 BLK movement were simple: achieve similar ballistics to the 7.62x39mm (AK) platform, develop an offering that delivers more “stopping power” when launched from a short-barrel rifle or pistol, and improve performance over 9mm offerings in the subsonic velocity band. Oh, and as an extra requirement, how about achieving the first two with the AR-15 platform we already have. Oh, and while you’re at it, make sure that standard AR magazine capacity is preserved.

The first goal seems easy, right? Why not just make an AR-15 upper chambered and barreled for the ubiquitous 7.62x39mm? For starters, that would be copying from our enemies and that strategy is kind of lame. More importantly, those heavily tapered rounds don’t feed from standard AR magazines. Even if you swapped the magazines for something more banana-shaped to accommodate the sloped Commie Cartridges, there’s still the problem of the AR’s vertical magazine well.

The 300 BLK cases (left) are cut-down .223 cases reshaped to handle the larger .30-caliber bullets.

What Is It?

The 300 BLK actually is a cut-down version of the .223 Remington. The brass case itself is chopped, reshaped, and expanded at the mouth to accommodate a .308 bullet instead of a .224-inch diameter one. Even though the bullets are longer and heavier than their .223 Remington peers, the overall cartridge length is still compatible with standard magazines.

The ramifications of that are important as we’ll see in a hot second. The case dimensions are identical as is the diameter of the case body. While the shape varies, the overall lengths are compatible, too.

Platform Differences?

Since the cartridges are so similar, not much surgery is required to convert a standard AR-15 firearm chambered in 5.56mm to 300 BLK. In fact, all you have to do is change the barrel. That’s a simple operation on an AR rifle or pistol. Once you remove the barrel nut, it just pops out for an easy swap.

Since the cartridge base is identical, the bolt and bolt carrier are unchanged. For the same reason, the magazines are interchangeable too. There are no modifications required to make a STANAG (NATO Draft Standardization Agreement) magazine work with 300 BLK cartridges. Oh, there’s one more benefit. Unlike “different” AR cartridge offerings like 6.8 SPC, capacity of magazines is also identical.

In loaded cartridges, much of the projectile is hidden inside the case. From left to right: 55-gr. .224, 77-gr. .224, 125-gr. 300 BLK and a 245-gr. lead subsonic 300 BLK bullet.

What About Energy?

I’ve chronographed common loads from both Springfield Armory SAINT pistols, so we can show examples of muzzle and downrange energy performance using real-life numbers. Do note that both of the test firearms are pistols, so the shorter barrels yield lower velocities than what you’ll see on the ammo box. The SAINT Victor Pistol 300 BLK model has a nine-inch barrel while the SAINT Edge Pistol 5.56 NATO sports a 10.3-inch barrel.

Downrange Performance?

One notable difference between 5.56 and 300 BLK is bullet weight. The “standard” 5.56 NATO projectile weighs somewhere between 55 and 77 grains. The most common supersonic 300 BLK loads use 110- or 125-gr. bullets. The subsonic version normally packs 220-gr. projectiles. This creates a tradeoff. The lighter bullets used by 5.56 NATO shoot flatter over distance but the fatter and heavier BLK bullets carry more energy downrange. The cost is more bullet drop. So, the right choice for you depends on what you care about more.

I ran the numbers with all four options to explore how velocity and bullet drop carried with both the 5.56mm NATO and 300 BLK choices. Both pistols were set up with a 50-yard zero.

I only included the subsonic 300 BLK data at 300 and 500 yards for consistency. We didn’t want that elegant table of data points to have big gaps in it, now did we? Although, assuming one can hold high enough to account for that 377-inch (that’s 31 feet!) bullet drop at 500 yards, the BLK is still moving at a respectable clip considering that it’s intended to “compete” against handgun calibers.

From left to right: 55-gr. .223, 77-gr. .223, 125-gr. 300 BLK and 220-gr. subsonic 300 BLK.

Subjective Factors

Bullet weight matters if you’re looking to maximize energy on target. While the classic kinetic energy measurement of ft-lbs. illustrates destructive power, the companion metric of momentum, measured in pounds-feet per second, communicates one object’s ability to move another. So, if you’re looking for a harder hit, the BLK offering might be right for you. On average, 55- and 77-gr. 5.56 bullets deliver 22 to 28 pounds-feet per second of momentum at the muzzle. 125- and 220-gr. 300 BLK projectiles carry 36 and 33, give or take.

Ammo cost is also something to consider. The 5.56 NATO world benefits from the sheer quantity of rounds fired. The more people shoot it, the more companies invest in serving billions and billions of rounds to the market. That drives down the cost per round for you and me. As a lower-volume caliber, you’ll generally pay more per round for both supersonic and subsonic 300 BLK cartridges.

The Net-Net

What’s right for you? That’s a bit like asking whether you prefer four-wheel drive or two. They both have pros and cons for their respective applications. Which one is right for you depends on exactly how you intend to use it. In the case of 5.56mm NATO and 300 BLK, both are suitable for defensive purposes. In the hunting world, 5.56mm is more commonly used for small game and varmints where fragmenting terminal performance is desirable. For larger game where penetration is more important, the BLK presents a better overall solution.

The bottom line is this: light and fast versus slower and heavier. Sounds kind of like the 9mm vs. .45 ACP debate, doesn’t it?

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Springfield Armory® recommends you seek qualified and competent training from a certified instructor prior to handling any firearm and be sure to read your owner’s manual. These articles are considered to be suggestions and not recommendations from Springfield Armory. The views and opinions expressed on this website are those of the authors and do not necessarily reflect the views and opinions of Springfield Armory.

Tom McHale

Tom McHale

Tom is a perpetual student of all things gun and shooting related. He's particularly passionate about self and home defense and the rights of all to protect themselves and their loved ones. As part of his ongoing learning, Tom has completed dozens of training programs and is a certified National Rifle Association instructor for pistol and shotgun. Tom is a professional writer by trade these days and has published seven books on guns, shooting, reloading, concealed carry, and holsters. In between book projects, Tom has published somewhere in the neighborhood of 1,700 articles for about a dozen gun and shooting publications. If he's not writing, you can probably find him on the range.

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