Why Subsonic .300 BLK has Better ‘Trajectory’ than 6.8 SPC

Last year I began a journey into the world of the .300 Blackout that I intend to continue here. In my last three articles ( I, II, III ) about the cartridge, I documented the theory of use, supersonic abilities, and weapons built to use this fantastic cartridge.

To summarize my prior work in a few sentences, the .300 Blackout has become a serious choice for today’s consumer in supersonic form. The cartridge is able to launch a 120-125gr projectile at speeds of up to 2400fps from a 16” barrel and can utilize specialty ammunition that can deliver match-like performance at ranges of 500 or more yards with velocities upwards of 2800fps. The .300 Blackout is a fully mature, modern cartridge that allows a full range of use with only a barrel change on a standard 5.56mm weapon system and a serious ballistic upgrade at close to medium range over existing 5.56mm options.

In these articles, I will be discussing the original ideas behind the cartridge and the nature of the .300 Blackout as a subsonic cartridge while looking at other modern rounds in the same playing field. I also put together a custom AR pistol to test both handloads and factory ammo and talk about the performance gaps between barrel lengths in this cartridge.

U.S.A. –-(Ammoland.com)- Here we will discuss how the trajectory of the .300 Blackout is better than that of other, faster rounds. We are, of course, not talking about ballistic trajectory, but rather the upward sales and popularity curve and the resulting technological gains in terms of subsonic bullet design. As you all probably know by now, the .300 BLK has taken the shooting world by storm. It has successfully outpaced the sales and popularity of other cartridges in the AR-15 world, such as 6.8 SPC, 6.5 Grendel, and many others, and done so with shocking gusto.

The reason for this is that, unlike rounds like 6.8 SPC, it offers the shooter a cartridge they actually want that is easily compatible with existing weapon platforms and can use off-the-shelf ammunition components with little to no issue. The Achilles’ Heel of the 6.8 SPC, for example, was specialty parts, magazines, and a shortage of good bullet options. The cartridge experienced commercial failure and is rarely used by anyone today except a few diehards as a result of the perfect storm of the 2009-2016 suppressor boom.

Where the .300 BLK truly shines is in its almost elastic versatility. There were never any good subsonic loads available for 6.8 SPC or 6.5 Grendel that could match those in the .300 BLK due to the fact that the smaller bore diameters didn’t lend themselves to loads at subsonic velocities. Why would this matter? The answer to this is twofold. The .300 BLK’s popularity came at a time when the suppressor market was exploding. Any supersonic cartridge could be suppressed, but not every supersonic cartridge could be easily made into an effective subsonic one.

The .300 BLK was readily adaptable to heavy bullets like the 220ge SMK, which at the time were available off the shelf where no comparable bullet was ready for 6.8 SPC or 6.5 Grendel. In order for a subsonic cartridge to be effective, it must have a bullet that carries enough mass under the speed of sound to do serious damage. Although it was possible to use heavy .277 bullets for the .270 in a 6.8 SPC, they could never equal the weight, and thus effectiveness, of .30 caliber bullets at the same speed. Instead of an advantage, it would be a deliberate handicap and a dangerous one at that.

This is a critical point in respect to the 6.5-6.8 cartridges. Their development was at a time when the idea was to improve upon the 5.56mm cartridge at distance.

The realities of the modern battlefield combined with the intense interest of the American public is what pushed the .300 BLK, despite being ballistically inferior at long range, into the spotlight and drove the others into abject obsolescence. In what was likely a first, the American public, usually concerned with long-range accuracy, decided with their wallets that a short-range, suppressor-friendly round was to stay and, as a result, serious design challenges were addressed in terms of bullet design.

What irks people that I know, especially those with a visceral hatred of the .300 BLK, is that it has a hard time fitting in with how they see rifles. The .300 BLK is a cartridge that can happily cross many performance barriers with the use of a single type of shotgun powder. Indeed, the same powders I used for my supersonic loads are also used in my subsonic loads by simply reducing the charge. It is troublesome to many staunch supporters of legacy calibers that a tiny .30 that uses shotgun and pistol powder at handgun velocities counts as a rifle in the sense of the word.

In terms of what a .300 BLK looks like in a subsonic sense, we need to look not at rifle cartridges, but instead at pistol rounds. Side by side, a 220gr subsonic load in .300 BLK is more akin to a .45ACP than it is to the supersonic form of the same cartridge. A standard .45 ACP can launch a 230gr bullet at speeds of about 800-950fps from a 5” 1911 pistol. My testing with 225gr subsonic loads in .300 BLK revealed that I could launch them at speeds of 950fps from a 7.5” barrel, making the two close in terms of speed.

Looking at the cartridge as a subsonic offering only, we are immediately confronted with some problems that make it, at least at close range, less effective than a .45ACP. How could that be? The .45 benefits from more frontal area, a myriad of bullet designs, proven, belt-carry pistol-sized weapon designs, and over one hundred years and billions of man hours of use. The new advancements in .300 BLK bullet design, like Hornady’s just-released SUB-X 190gr load, share more with pistol rounds like .45ACP than that of rifles and for good reason, but more on that amazing new round later.

The main concern with subsonic loads is that they share a single trait: their limits. Why I treat the .300 BLK in subsonic form as a specialty pistol load is that it has all the same limitations as pistol cartridges in that available bullet mass restricts necessary bullet speed. Because we need to have a bullet moving under the speed of sound to make it subsonic, we need the bullet to perform to our desired result within that limit. Taking a full size AR rifle and making it have the effectiveness of a pistol seems ludicrous and rightfully so. This is where the design of bullets becomes so critical.

The market for ammo that functions in a rifle, while flying at pistol speed, is fairly new. There are many radical designs available today that address this issue. First is rounds like the SIG 220gr OTM, despite being extremely accurate and high-mass, offer no expansion and instead rely on said mass to be effective. Penetration ability of these bullets is evident, with most of my test bullets being able to go through almost anything besides hardened steel plate during testing. They are not, however, recommended for hunting.

The other means of effectiveness is expansion, which is a hard thing to rely upon at low velocities, but new advances have corrected that. I have tested lots and lots of subsonic ammo in a variety of mediums over the last few years and have discovered that, especially in a hunting scenario, that traditional is better. Old-school softpoints and straight-up hardcast lead in the 200-240gr range often outperform new alloy-only bullets, which tend to shatter while not expanding. I have noticed this problem for some time and wondered why nobody else did, but it turns out Hornady took notice.

Hornady’s SUB-X 190gr load boasts a traditional design similar to the bullets found in their Critical Duty and Critical Defense handgun ammo and bests FBI protocol in terms of penetration and weight retention. I tested this load on barriers and glass out to 100 yards and found that the results were equal at ranges inside 25 yards. Recovered bullets that went through glass and material displayed remarkable toughness and maintained 70-80% of their mass. This is an incredible ammunition and would easily cross over to hunting should it be called upon.

Hornady has, without a doubt, raised the bar for which subsonic .300 Blackout is judged.

A distinct advantage at distance when using .300 BLK instead of a traditional pistol cartridge is, despite nearly equal bullet weight, superior energy retention and accuracy. The things that make a .45ACP effective at close range kill it at distance. The aerodynamic rifle bullets maintain their speed far better than short, fat pistol bullets and thus carry greater energy over distance.

I was able to routinely put ten shots of Hornady 190gr SUB-X into 2” at 100 yards. The load generated 1006fps over my Oehler 35P chronograph from a 7.5” Faxon barrel. Testing SIG SAUER 220gr loads proved better in terms of accuracy at that distance with a velocity of 990fps. This meant that I was able to, at 100 yards, deliver eyeball accuracy with virtually no recoil and still have more power on target than a .45ACP point blank. Not bad, huh?

The fact that there is so much speed, and thus energy, retained by the .300 BLK at subsonic velocities make it, with practice, a lethal round at 200 yards, but there are limits to that.  Many hunters rely on energy as a measure of load performance, but it can be deceiving. The subsonic .300 BLK in almost any form has less than 500 foot-pounds of energy at the muzzle, which is again right in the ballpark of .45ACP.

The problem with this is that most hunters consider 1000 foot-pounds on target to be the minimum, something that even .45 ACP can’t match despite being a very popular hunting pistol cartridge. I don’t look at it in terms of foot-pounds these days. Why? I think that bullet technology has arrived to where effectiveness is a function of mechanics rather than retained energy, meaning that if we can have a bullet that, at 100 yards, can pass through 15” of gel, pine boards, and leather while expanding reliably at around 1000fps, we have gone beyond the guidelines into new territory.

The function is there, but the classic numbers don’t match. I think we will begin looking at the new generation of hunting bullets the way we have classified long range bullets in the G7 rather than G1 drag coefficients. We need a new criterion to match our new technology and I think that is on the horizon.

So what does all this mean for you, the discerning shooter?

In this article and my previous three, I have detailed what makes the .300 BLK more popular and effective than its competitors, talked about available options and ammunition choices, and about the limitations of the cartridge. What we have with the .300 Blackout is a true do-all cartridge for the modern consumer. You can, with some limits, have a weapon that can be easily suppressed, maintain effectiveness and accuracy in a barrel under 8”, load it with ammunition that runs anywhere from 800fps all the way up to 2800fps, and take full advantage of a plethora of available reloading options.

In my next article I will discuss the building of a .300 Blackout with a 7.5” barrel and the necessary things to consider when making handloads for subsonics out of such short barrels.

Special thanks to:

• Hornady
• Brownells
• Faxon
• V7 Weapon Systems
• Maxim Defense
• Magpul
• Midwest Industries

About Josh Wayner:

Josh Wayner has been writing in the gun industry for five years. He is an active competition shooter with 14 medals from Camp Perry. In addition to firearms-related work, Josh enjoys working with animals and researching conservation projects in his home state of Michigan.

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