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6.8 SPC Adds Muscle to Ruger’s SR-556

by Patrick Sweeney   |  December 16th, 2010 1

The 6.8mm SPC chambering gives Ruger’s newest SR-556 plenty of punch.


I’m standing behind the trunk of a dead tree, carefully watching the target downrange. On the word “Go!” I snap the buttstock to my shoulder, brace against the tree and press the trigger. The sound of the shot is quickly followed by the return sound of bullet on steel. At the hit, I race off to the next shooting location and skid down to a kneeling position, braced against the side of available cover. A quick press, a return hit. Only then do I realize two things: The sound of the hit on steel is a lot more authoritative than it had been a few minutes ago, and it is getting hard to breathe.

The reason for the latter is simple: I’m not a kid anymore, and I’m racing through the Scrambler at Gunsite, where the range I’m on is just short of a mile high.

And for the former, the sound is louder because I’m using a factory prototype SR-556 in 6.8 Remington SPC. While I did not match my previous best time on the Scrambler (the last time I shot the course, I was 20 years younger and trying to match the time of a SEAL master chief) I did not embarrass myself.

Alas, since it was a factory prototype, I had to leave the 6.8 with the Ruger engineers and wait at home for a 6.8 test rifle to show up. It did, quite quickly, and with all the extras that Ruger includes with each rifle.


With the SR-556 6.8 you get a more powerful but perfectly controllable piston driven AR that can be pressed into service for a wide range of duties.

The SR-556 in 6.8 is exactly what you’d expect, if you have any familiarity with the SR-556 in 5.56. However, not everyone does, so we’ll take a brief moment to bring you up to speed.

The SR-556 employs Ruger’s two-stage piston, which has four gas-flow settings (it arrives set on “2,” and you will probably never have need to change it to any other setting), and the piston system is housed in a gas block on the barrel that is also railed for a front sight. Onto that block, Ruger has bolted on a Ruger-badged folding front sight, which pairs with a folding rear sight–both from Troy.

The barrel is a cold hammer-forged tube made from 41V45 steel–hammered, threaded and fitted by Ruger. It is chrome-lined, and you’d have to work very hard to make or find a barrel that was tougher.

As for accuracy, you’d have to spend a bucket of money to get one that delivered more. At 16 inches and change, the barrel is long enough to be kosher everywhere, but some jurisdictions will grump about the threaded-on flash hider. The hider is very much like the design Ruger used on its law enforcement-only Mini-14 models, and while it does not look mil-spec, it works as well as any and better than most.


Other than the 6.8 SPC chambering, the SR-556 is a standard, mil-spec AR, although it comes with nice extras such as a Troy folding rear sight.

The only clue to the different caliber is to be found on the barrel itself, where at the 12 o’clock position, in front of the gas regulator, it is marked “6.8mmRemSPC.”

Despite being relatively new, the 6.8 has already gone through the process of improvement, to include a different chamber spec, and the 6.8 cognoscenti will extol the virtues of the “Spec II” chambering for as long as you’ll sit still and listen.

Basically, the original and the “II” chambering compare exactly as the .223 Remington and the 5.56×45 NATO do. The latter of each pair features a leade that has a longer freebore and a less-steep onset of the rifling. Rifles in the 6.8 Spec II chambering also feature a slightly slower twist, which takes the edge off of the pressure spike. The advantage of the newer chambering is the potential for greater velocity while controlling pressure.

Specifications – Ruger SR-556 6.8
Type: piston-driven AR
Caliber: 6.8mm Remington SPC
Capacity: 5- and 25-round CProducts detachable box magazines
Barrel: 16.12 in., 1:10 RH twist
Overall length: 323⁄4 to 36 in.
Weight: 73⁄4 lb.
Finish: manganese phosphate and hardcoat anodized
Stock: M4 telescoping w/Hogue Monogrip
Sights: Troy folding front and rear
Trigger: 5 lb. pull as measured
Price: $1,995
Manufacturer: Ruger, ruger.com

Almost as soon as I had digested the news of an SR-556 in 6.8, the question came up, Which chamber spec? The answer: the original. Why not the II chambering? Simple–it is not a SAAMI-spec chamber.

You may want to be irritated at Ruger for choosing the original, but you cannot e
scape its logic. First, it would run contrary to Ruger’s customary practice to adopt a non-SAAMI spec chamber. Second, if it was going to adopt a Spec II chamber design, it would have to choose one from among myriad competing boutique chamber designs. Hence Ruger went with the industry-accepted and adopted chambering.

As a piston-driven rifle, the carrier differs from the original AR, direct-gas design. Ruger manufactures its chrome-plated carrier with an integral thrust shoulder on it for the piston to work against. Up front, the bolt, extractor and cam pin are also chrome-plated, and at the rear the carrier has an anti-tilt collar machined onto the carrier tube.

The handguards are railed, made specifically for Ruger, and the handguard is pinned (and robustly so) to the upper receiver. What this means is that if you have a fondness for some other rail design, you will have your work cut out for you to swap this for yours.


The rifle sports a railed fore-end on which a Troy folding front sight is mounted. The gas block houses the adjustable, two-stage piston system.

However, if you can let go of that affectation, you’ll find that the fore-end is well-proportioned, and by pinning the two together Ruger has made a hell-for-tough rifle.

It does mean that the internals of the piston system are inaccessible to you. You can remove the piston itself, as well as the regulator cap, but the transfer rod and spring are not user-serviceable items.

As for the rest of the SR-556 in 6.8 (and they really need to come up with a shorter, snappier name for this rifle) it is a standard AR-spec rifle. The lower is filled with mil-spec parts, and the dimensions are all correct, so if you feel the need, you can swap out the perfectly good mil-spec parts (the trigger pull on both the prototype and the test rifle I received were quite nice) and install some other trigger system.

Me, I would not change it unless I was using the 6.8 for one of two purposes. If I was using it as a varmint rifle, I’d prefer a match trigger for the last bit of accuracy I could wring out of it at distance. If I was using it in competition, I’d want something a little lighter in pull just so I could speed up my split times and thus shoot stages more quickly.

You’ll find the same situation with replacement magazines, rail covers, sling, buttstock and so on. However, you may find that replacements are not all that necessary, as the CProducts magazines, (one five-round and two 25-round included) the Troy rail covers and sights, the M4 buttstock and the Hogue Monogrip pistol grip are all top-notch. If you really want to change things, go ahead. Just don’t kid yourself that you’re making huge improvements in fit, function or anything but personal preference.


The SR-556 6.8 features a nonslip Hogue Monogrip for better handling and control.

Since it was a loaner rifle, I suppressed my normal desire to start changing things and shot it just as it arrived. For accuracy testing, I used my Leupold Mk 4 MR/T scope in its LaRue mount to wring out the best groups. For testing in drills, I took the scope off and mounted an EOTech EXPS3 and thrashed the gongs and falling plates on the 100-yard range until I was satisfied that the rifle was not going to quit on me.

As for accuracy, this particular rifle showed a great fondness for Hornady’s 110-grain V-Max. It was almost as fond of that company’s boattail hollowpoint, which was plenty accurate for all but tiny targets at max distance.

It was not particularly impressed with Remington ammo, but my stash of 6.8 in green and yellow boxes dates from the earliest production of 6.8, and newer ammo is bound to shoot better.

I kept the rifle on the “2″ setting for the gas regulator, and all loads fed flawlessly. The empties ejected a little behind and well to the side.

The gas settings of the Ruger piston system are there simply to give you some adjustment if you run into a batch of ammo (or firing conditions) where you need just a bit less, or a bit more, gas.

The lowest setting is “Off,” which is for people who want to use their rifles as a straight-pull bolt action and not a self-loader. Also, the piston, while it does not blast the receiver with hot, dirty gases, does not make the gases go away. You really should strip the gas system and clean it now and then, unless you really want to carbon-weld your regulator in place.


The bolt carrier features an integral piston thrust shoulder, and the rear of the carrier has a machined collar to prevent tilt.

As far as recoil is concerned, it really isn’t much. Yes, on paper the rifle should deliver a lot more thump to your shoulder, but it doesn’t seem like it. A 5.56–launching a 55-grain FMJ at 3,150 fps–has a power factor (momentum-recoil) of 173. The 6.8 with a 110-grain bullet at 2,475 has a power factor of 272. That should mean a shoulder thump half again as hard, but for some reason it doesn’t seem like that much.

Now, I would not hand the SR-556 to a brand-new shooter and tell him or her, “It won’t kick.” It will. But if you’re used to a 5.56, the 6.8 is not that much of a jump.

If you want a defensive rifle with more oomph than a 5.56, then this is it. If you need a rifle that can be used for hunting deer or similar game, then–depending on your state’s regs–all you need do is slap in the five-round magazine and you’re set.

The SR-556 in 6.8 is available only as a complete rifle. In talking with the engineers and management of Ruger, I found that the biggest discussion on the 6.8 project wasn’t whether to do it or what features it should have–or even if it should be called the SR-68 instead. No, the big question was whether to offer it as just an upper or as a complete rifle.

For now, it is a complete rifle. However, the new Ruger is not like the old Ruger, and if the market demands uppers alone, then Ruger will respond. Me, I’m not worried. I have found that the allure of spare uppers to swap across one or two lowers is one that fades quickly. It doesn’t take too long for new AR owners to decide to stop the swapping and to settle each upper onto its own lower.

At the suggested retail
of close to $2,000, the cost of the gun may seem a bit much. However, there is suggested retail and there is actual retail. And when you factor in the extras–the magazines, sling, rail covers on a railed free-float handguard, iron sights–the total is reasonable in the current AR market.

Accuracy Results | Ruger SR-556 6.8
6.8mm Rem. SPC Bullet Weight (gr.) Muzzle Velocity (fps) Standard Deviation Avg. Group Size (in.)
Hornady V-Max 110 2,502 6.5 1.00
Hornady BTHP 110 2,547 7.3 1.25
Remington OTM 115 2,451 21.4 2.25
WARNING: The loads shown here are safe only in the guns for which they were developed. Neither the author nor InterMedia Outdoors, Inc. assumes any liability for accidents or injury resulting from the use or misuse of this data. NOTES: Accuracy results are averages of four five-shot groups at 100 yards off a sandbag rest. Velocities are averages of five shots measured on a CED M2 chronograph centered 15 feet from the muzzle. Abbreviations: BTHP, boattail hollowpoint; OTM, open tip match.

  • Dan

    It's a great rifle to shoot I love it but if your going to do a bunch of target shooting be aware rounds average about $1.00 a piece!

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