Ask Foghorn: 6.8 SPC versus 300 BLK?

Reader Erik writes:

I’ve been reading many articles re. defensive rifles/ammo and it seems I’ve noticed your fondness for the .300 BLK round.  I am impressed as well but the overall lack of its use and the increased cost/round concern me.  I know that round for round 6.8 SPC ammo is about the same right now, but given the increased popularity of the 6.8 with many military agencies, I am betting that over time the round will become more available (just like the 5.56 is now) and will become less expensive. Would appreciate your thoughts on this.

I get this a lot, actually. And its a valid question — what makes 300 BLK so much better than the other calibers available for the AR platform? And is it actually better? So here’s the truth…

It depends on what you want to do.

I’m going to break this into three categories, encompassing everything a shooter should consider when choosing a caliber: performance, compatibility and availability.


The 6.8 SPC and 300 BLK cartridges were designed for two very different jobs. 6.8 SPC was designed to be a longer range cartridge, while 300 BLK was designed to be a shorter range cartridge that worked well with a silencer. I interviewed Kevin Brittingham, founder of AAC (and godfather of the 300 BLK cartridge) and he had this to say about his round:

These were our original requirements for this caliber: Muzzle energy has to equal or exceed the AK-47. .30 Caliber projectile. Use unmodified 30 round magazines to full capacity. Use unmodified AR-15/M-16/M-4 bolt. Gas impingement system. Shoot super and subsonic. And one thing that was nice, but was not a ‘deal killer’, was non-adjustable gas system. Cycle all four ways – subsonic suppressed and unsuppressed, and supersonic suppressed and unsuppressed.


It’s not one of the 6.5 or 6.8 or whatever is the flavor of the month where it’s a do-all, kick-ass… From a military standpoint, here is the advantage (which we discussed earlier): all we need is to change the barrel and get the ammo. Non-adjustable gas system… And that’s a huge thing. Nobody works with silencers more than us. Every time I’m working with a company on something, whether it’s LWRC or Colt or FN, whatever it is, if it’s got an adjustable gas system we get halfway through the test and we’re like “holy shit, we didn’t adjust the gas system.” It’s not something that’s really intuitive, you gotta think about having a suppressed system or unsuppressed. If you can get away with a non-adjustable it’s always best. And we’re able to achieve that, and it’s because we did the whole gun; we did the ammo and the silencer so it all works together.

So, 300 BLK was designed to fill a specific role. It was designed to be a big, heavy round that can be quickly changed from subsonic to supersonic and suppressed easily.

6.8 SPC, on the other hand, was never intended to be quiet. The relatively small projectile was designed to add a little more mass to the round while maintaining that zippy velocity that gives 5.56 ammo its punch. And thanks to that extra velocity, the 6.8 SPC does indeed perform better at longer distances.

Out at 500 yards, 300 BLK drops a whole extra 30 inches compared to 6.8. But to 200 yards, the difference is less than two inches.

That higher velocity also means that the 6.8 cartridge has a higher muzzle energy than the 300 BLK, clocking in at about 1,694 foot pounds for the 115gr variant to 300 BLK’s 1,360 for their 125gr offering. And the 6.8 SPC’s performance only improves with added weight. (By the way, 5.56 NATO 55gr comes in at 1,188 foot pounds.)

So, in short, if you really want the most muzzle energy and long distance performance then 6.8 is the way to go. Actually .338 LM is the way to go, but that won’t fit into an AR-15 magazine.


To me, the entire point of having a funky caliber in the AR-15 platform is to be able to switch quickly and seamlessly from one caliber to the other. I should be able to swap uppers and be good to go. And, in general, that’s possible with both calibers. But one caliber really does it better.

The 6.8 SPC cartridge is designed from the .30 Remington cartridge, meaning that it will not work with a standard AR-15 bolt. You need a slightly larger bolt, which means less material on the bolt face to contain the force of the escaping gasses and a higher probability of failure (less material + higher muzzle energy…). And when it fails, you’ll need one of your special snowflake bolts instead of any old standard AR-15 bolt.

Speaking of special snowflake parts, while the 6.8 SPC will fit in an AR-15 magazine, its actually too fat to work at full capacity. Most manufacturers appear to warn users to only load five or six rounds to avoid bulging the magazine. Magpul is coming out with a new magazine specifically designed for 6.8 soon, but the rumors are that it will only work in LWRC’s special purpose-built 6.8 lower receiver and not standard ARs.

The headache doesn’t stop there, though. There are, by my count, four incarnations of the 6.8 caliber. One is the SAAMI spec, which is a lower pressure load. Then there’s SPC II and 6.8×43, both of which have different chamber pressure ratings. And finally there’s Noveske’s Mod1 design, which apparently allows the highest pressures and best performance — but its proprietary, and not cheap. In short, you need to double check that you’re buying the caliber you want before you drop the coin on that new upper, and ensure that you’re getting the right ammo.

300 Blackout is much simpler. There is only one flavor, which is an open and freely available SAAMI spec that is already rated to a higher chamber pressure than its wildcat predecessor (the 300 Whisper). Which means that if you get a 300 BLK barrel from any manufacturer you can rest assured that its going to take any 300 BLK ammo you throw at it and more.

300 BLK also removes a lot of the equipment headaches. Since it is made directly from 5.56 brass and was designed specifically to work in standard AR-15 magazines, it works flawlessly with standard AR-15 bolts and magazines. There are no issues with feeding, no issues with magazine capacity, and spare parts are abundant since the only real difference is the barrel.

So, if you’re looking for a simple drop-in change for your AR-15 that works with all of your existing gear and spare parts then 300 BLK is the way to go.


Having a gun chambered in a kickass caliber doesn’t matter if you can’t find any ammo for that gun. And, right now, 6.8 SPC is winning.

6.8 SPC is just about everywhere — the only place I still can’t find it is Wal Mart. Its been about a decade since the first 6.8 rounds started appearing, and so just about every manufacturer has a load available and ammo in stock. And, thanks to the community of shooters using 6.8 SPC already, there’s no doubt that this round will continue to be available in the future as well.

300 BLK is the new kid on the block. It hasn’t been adopted en masse by military or law enforcement just yet, so there’s no milsurp loads available. But while the “professionals” might not have adopted it, I’ve seen more and more hunters adopting the round as a way to quickly change from their 5.56 M4gery to their hunting rifle. Especially down here in Texas where hog hunting is huge, the ability to switch between subsonic and supersonic and suppress the ammunition effectively is a HUGE plus. That use case has led to a huge demand for guns in the new caliber, and these days just about every manufacturer makes a gun in 300 BLK.

While the growing number of 300 BLK owners bodes well for commercially available loads, one of the best things about 300 BLK is that it will always be available as long as there’s new 5.56 ammo being made. Well, for handloaders at least. Since the 300 BLK round can be made directly from 5.56 brass you will always be able to make ammo for your gun even if the manufacturers stop making it. 6.8 SPC, on the other hand, uses the obsolete 30 Remington cartridge that hasn’t been produced commercially en masse in decades. So if manufacturers stop making 6.8 SPC for some reason, unless you have a stockpile of old brass you’re kinda screwed.

In the end, though, its a tie for availability. 6.8 SPC is already widely available, and 300 BLK has the backing of one of the largest ammunition manufacturers in the world and the largest firearms company (Freedom Group) known to man. Putting aside my conspiracy theory about 300 BLK being Freedom Group’s Apple-like walled garden attempt, that’s a lot of resources being thrown at making 300 BLK a viable caliber for the average shooter.


Like I said at the beginning, it all depends on what you want to do.

If all you’re looking at is muzzle energy and long distance performance, then 6.8 SPC is the clear winner. Its a field tested and battle proven cartridge with a huge following that can put animals down at longer distances than 5.56 NATO. And if you’re comfortable with the slightly more complex world of parts and ammunition, then go for it.

But if you’re looking for something for hitting living animals up to 200 yards away, does better than 5.56, uses all your existing gear, can swap easily from supersonic to subsonic and is easily suppressed, then 300 BLK is the winner. It may not be a great long distance round, but the compatibility with existing gear really was the feature that sealed the deal for me.

I like 300 BLK. But then again, you knew that already. And really, if I wanted something harder hitting or for longer distances, 7.62 NATO would be my choice.

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