A History of AR Cartridges

.30 Remington

The 6.8 Rem SPC is derived from the old .30 Remington case, shortened, necked-down and made to work in an AR.

 

6.8 Remington SPC

The “six point eight” erupted on the scene as the replacement for the “anemic” 5.56. The idea was to use a case with more volume than the 5.56, but not one that required a wholesale redesign of the rifle. The case settled on was the old .30 Remington, with some changes.

Now, this is not anything new. Back in the mid-1980s, I was fiddling around with new designs. One I came up with was a 25mm grenade for a self-loading grenade launcher, for use in the military. I wanted to come up with something besides the single-shot M79, or the bulky and awkward M203.

So, I did some thinking, made some drawings and turned sample cartridges out of aluminum rod. However, lacking both a loading lab to make sample test shells for further experimentation, and the licenses to do such work, I had to leave it at drawings and solid-aluminum dummies. However, while I was doing that I happened to have not one, but two customers’ rifles chambered in .30 Remington come through for work.

In the course of repair and test-fire, I had to track down some .30 Remington ammo. As I was looking as the ammo, I happened to have a 20-round AR magazine nearby. Just out of curiosity, I snapped the loaded round into the magazine. Hmmm, pretty close, but too long. I figured I could make changes, and perhaps even re-barrel a rifle. A glance at my elderly lathe made it clear I had not the equipment to hold the tolerances to turn down a barrel or barrel blank and fit it to an AR.

There was also the matter of timing. Back then, no one would have been interested in a replacement cartridge for an AR that didn’t do what a .308 did. If I couldn’t make “Major” there was no point to the experiment. The .30 Rem Short (as I had mentally named it then) had no chance of being boosted to Major, not with the powders we had back then. So I shelved the idea. Now, I make no claim to being first, only or the cleverest on that subject. I’m just pointing out that it is durned difficult to come up with something that is truly new.

The 6.8 was not meant to make Major. It was meant to produce the most “oomph” out of an M4 carbine, with the least amount of modification to the package, and the most commonality with existing gear. As such, it requires a new barrel, a new bolt (which is essentially the old bolt with a bigger bolt face) and new magazines.

The original plan, I’m sure, was to make it work in existing magazines. Well, that just couldn’t happen. What I am sure of is that if you were willing to invest enough computer simulation time, you could come up with a 6.8-ish cartridge that stacked and fed from unmodified AR magazines. I’m also sure that by the time you got done modifying the cartridge case to permit such feeding, you’d have lost enough case capacity that you didn’t have performance any better than the 6X45.

So, the 6.8 got new magazines. And what is the performance that makes new bolt, barrel and mags worth it? At the low end of weight, we’re talking a 90-grain JHP at 2800 fps. Moving up, the “sweet spot” seems to be in the 110 grain range, where a 110 JHP or OTMcan be pushed just short of 2600 fps. Now, for those who are accustomed to a screamer 5.56 load like the XM-193 (a 55-grain FMJ at 3200 fps) or the Mk 262 Mod 1 (a 75- or 77-grainer at 2800 fps) the 6.8 may not seem like much. But with the 6.8 we get back that corner of the performance envelope that the 6X45 gave up. You’re now pushing a heavy bullet fast enough that it will upset, or, when it tumbles, does good work moving sideways.

Now, as with the .223 vs. the 5.56, there is the original, and the later 6.8. The original was designed as a collaboration between the Special Forces NCOs who had the idea and the Remington engineers who did the detail work, drawings, etc. The .223 differs from the 5.56 in that the lead-in to the rifling on the .223 is shorter and steeper than it is on the 5.56. The reasons are thus: the .223 is meant as a varmint cartridge, and there accuracy is prized over all else. The 5.56 is a combat cartridge, and reliability and pressure control are prized. So, the longer freebore and gentler leade of the 5.56 allows for heavy bullets (like tracers) and for a dirtier operating environment.

The original 6.8 was designed more along the lines of the .223. Soon after, experimenters changed it. They lengthened the freebore and the leade angle was made more gentle, plus one more change; rifling twist. The original twist is/was 1:10; the new uses a 1:11 twist. With a greater freebore, gentle leade and slower twist, the 6.8 II is better able to handle pressure than the older design. It also makes the bullets just a bit closer to unstable, although still accurate, and this enhances terminal ballistics. Well, with all that, the government didn’t adopt the 6.8. In fact, the NCOs who pushed it got into hot water.

You see, good ideas are valued by large organizations as long as good ideas come as a result of the system. Good ideas that are not a result of the system are heretical and must be quashed.

Is the 6.8 a good idea? You bet. As long as you are willing to make the investment in the new gear, it is a very good idea. Magazines are now readily available from CProducts, PRI and others. One you won’t see, or so the guys there tell me, are PMags in 6.8. The fatter cartridge just won’t stack properly inside a magazine tube made of polymer. At least, not with the thickness needed for durability. Sure, they could make it thinner, but who wants a fragile polymer magazine?

Magazines fit in mag pouches, reloading presses work on 6.8 just as they do any other cartridge, and the bullet diameter is a common one, so no problem there.

In all fairness, one big problem for the military is how much commonality there it. For us, the fact that it is all so close is a big advantage. For the military, it’s a big disadvantage. You see, you can load 6.8 in regular mags. Or you can load 5.56 in 6.8 mags. And either will fit in the other’s mag well. I haven’t seen a 6.8 try to digest a 5.56, but I’ve seen a 5.56 try to chamber a 6.8. So, if the military is going to adopt it, they have to make a clean sweep.

So, if they want to try the 6.8, to see how it works, they have to make sure that the troops going into Carjackistan are all armed with 6.8s and nothing but 6.8s. and anyone who comes to support them must have 6.8s. The supply system has to be hyper-vigilant about ammo, otherwise a chopper will arrive at a dusty FOB, kick out a pallet of crated ammo, and take off before the locals get a chance to have a go with their RPGs. And the troops will find the ammo is 5.56, not usable in their 6.8 rifles.

Worse yet, the Army can’t just re-barrel existing M16/M4s. The system isn’t set up to allow a rifle, M16A-whatever, clearly marked on the receiver as a 5.56, to be chambered in anything else. There would have to be, at the very least, a new designation, and the altered rifles so marked. It would be better if they were new ones, cosmetically modified in some way to make them clearly different.

For the military, changing to 6.8 is not a “bolts, barrels and magazines” change, but a billion-dollar cost. For what? A bit more terminal effectiveness? That’s what radios and artillery are for.

3 thoughts on “A History of AR Cartridges

  1. gunslinger454

    There are a few other good AR15 rounds out there as well. As ‘theken101′ mentioned above, there is the .300 AAC Blackout which, if I’m not mistaken, is little more than a renamed & SAAMI approved version of the much older .300 Whisper wildcat round. Either one when loaded with a 110gr-115gr bullet basically replicates the ballistics of the 7.62x39mm round in a cartridge that is far more compatible with the AR15 platform, requiring only a new barrel.

    For the varmint hunters out there the .204 Ruger is a superb cartridge for sniping ground squirrels & prairie dogs, and, like the .300 Blackout, requires only a new barrel to work in an AR15. It’ll spit out a 32gr bullet at over 4,000fps and excels at exploding those prairie destroying little rodents at long range (as long as you can deal with the wind).

    One of the new cartridges that I find very interesting comes from Olympic Arms and was designed to exceed .308 Winchester ballistics in the AR15 platform. They call it the .300OSSM–Olympic Super Short Magnum–and it is essentially a Winchester Super Short Magnum case necked up to .30 caliber. Though magazine capacity is low, the cartridge holds a lot of promise for hunting & long range shooting with the AR15 platform.

    Then of course there’s the true big bores: the .450 Bushmaster, .458 Socom & .50 Beowulf. Any of the three will take any animal that walks on the North American continent, and they make dandy hog hunting rounds!

    My personal favorites are the .223/5.56, the 6.5 Grendel & the .458 Socom. The .300OSSM has also peaked my interest!

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