Ruger's SR-556/6.8 Part I

The 6.8 SPC cure for cartridge deficiency anemia

When Ruger released the SR-556 as their entrance into the very dynamic "Modern Sporting Rifle" market, it was obvious they had no intention of going in cheap, as defined by price or quality and design of equipment. Not only is the SR-556 a standout in regards to features like the gun's gas operated, piston driven actuation system, but it is also a standout because of the inclusion of popular, high end, third party components... like Troy Industries BattleRails™, Folding BattleSights™ and hand protecting Picatinny RailGuards™.

Accessories are ever changing in the world of the AR, as are the names of the companies perceived to have the best ideas. With the SR-556's modular approach, Ruger is able to concentrate on producing the best core firearm and associated performance, while still providing customers with the best and newest accessories.



For those fond of box scores...

Ruger SR-556
Manufacturer Sturm, Ruger & Co.
Model SR-556/6.8 #5909
Finish Manganese Phosphate/Hard coat Anodized
Stock Poly - Collapsible
Handguard Floating - Multiple Picatinny rails
Action Gas Operated, Piston Driven Autoloader
Caliber 6.8 Remington SPC
Magazine Capacity 5 & 25 Rounds
Barrel Length 16.1"
Rifling 1:10" RH 6 Groove
Overall Length 32¾" Collapsed - 36" Extended
Length of Pull 10¼" - 13½"
Trigger Pull 8 lbs 3 oz
Weight - W/O Mag 7 lbs 11 oz
MSRP - Rifle and Kit $1,995
Discount Retail $1,500

RealGuns prior coverage of the 5.56 NATO version of the rifle can be found at "Ruger's SR-556 -What comes after an AR-15?".

This version of the SR-556 is the same rifle in form, fit and function, only adapted to the 6.8 Remington SPC (0.277") cartridge. There are six parts that differentiate the two models: Barrel Assembly, Regulator, Crush Washer, Flash Suppressor, Bolt, and Extractor. All items accommodate either the physical size or gas volume / pressure of the two cartridges. The 6.8 SPC version has a slightly slower rate of rifling twist.

A few points to consider...



The Ruger SR-556 as received is ready to go. Differing slightly from the SR-556 5.56 NATO version pictured above, the 6.8 SPC version Ruger retail carton contained two twenty five and one five round magazines. Both models include the soft case and rail covers.

External look and operating overview...



The flash suppressor (1) ahead of a peel washer is pretty much standard fare and the same as the flash suppressor on the current Mini-14 Tactical. The front sight mounted above the gas block (2), is a Troy Industries adjustable folding battle sight with HK Globe type brush guards. The gas regulator and gas block (3), is an all steel assembly with four indexed settings to adapt the gun to various types of shooting and ammunition. "0" position blocks all gas from entering the system and renders the SR-556 a single shot that must be manually cycled, Positions 1 through 3 allow progressively more gas flow with position 2 being the typical for factory ammunition.

Folding battle sights...



The Troy Industry sights are a good selection. Easy to install and remove using the gun's Picatinny rail system. They are very solid. Popped up for use, the front sight offers elevation adjustment in 1 MOA click increments, the rear sight provides 1/3 MOA incremental windage adjustment. Both sights fold to a compact 1/2" height to place them out of the way of additional optical or electronic sighting systems that might be installed by those of us with 5 MOA eyesight.

A rail...y good way to mount stuff


The hard anodized and Teflon coated aluminum floating handguard (4), incorporates four Picatinny rails - 3, 6, 9, and 12 o’clock positions, each location marked with white alphanumeric identifiers to assist when installing and reinstalling optics and other accessories.

The top rail blends into the Picatinny rail that is integral to the gun's receiver; the top rail runs the length of the gun. A set of rail covers are supplied for three sides to provide a comfortable handhold when accessories are not installed at these sections of rail.

The layout of controls and placement of locating pins are all AR standard, so upgrade and newly developed bits, pieces and accessories can be added as the gun's owner desires. As an example, an oversize cold weather trigger guard, a cushy palm swell grip, other standard AR magazines, and stocks suitable for differing applications can easily be installed. The exception to universal parts fit would be the gun's handguard which is made to clear the Ruger's adjustable gas block and to allow passage for the transfer rod and spring assembly that replaces the gas tube found on a conventional gas operated AR. This is also a factory fit part and not intended for customer removal. As the SR-556's handguard is of a very current and popular configuration, I couldn't see the need to make a change to this part in the foreseeable future. Ruger also offers a complete 6.8 Remington SPC upper receiver assembly that customers can use to switch calibers if they own a small bore SR-556 or they can match it up with any of their favorite non-Ruger AR15 standard lower receiver assemblies.



The Ruger has a well made collapsible stock that is adjusted by pushing up on the lower lever. Lock up is positive, the stock feels stable in use and the shooter may select one of six positions. As an old Air Force vet, I have no idea how to shoot the gun with the stock in any position other than fully extended

The buttstock includes a sling swivel, the handguard does not, however accessory sling swivels that attach to a Picatinny rail are easy to come by. Brownells offers a large selection of this type of hardware at very reasonable prices.

Maintenance takedown...



Take down for routine cleaning and maintenance is easy. Pull out the charging handle, cock the hammer and rotate the safety to the "safe" position. The rear takedown pin is pushed out to the detent, I used a Delrin AR15 take down tool, then the upper assembly can be tipped down and the bolt carrier and charging handle can be removed. Push out the forward pivot pin, left to right just like the takedown pin, and the upper receiver can be removed from the lower.

The center of the SR-556 universe...

Conventional AR-15 type firearms have a gas block fixed some inches aft of the firearm's muzzle, over an open orifice drilled in the gun's barrel. A hollow tube runs from the gas block down the length of the barrel, under the cover of the gun's handguard, through the face of the receiver and into the tubular receptacle of the bolt carrier key that sits atop the bolt carrier. When the firearm is discharged, gas is bled off from the barrel into the gas block, then into the gas tube and down to the bolt carrier key where pressure forces the bolt carrier open and cycles the gun. The gas is hot and contains powder residue which, combined, pumps miniscule debris into the gun's receiver and elevates operating temperature. Additionally, on conventional ARs the gas flow is of fixed regulation and may be sensitive to ammunition power levels - lower pressure ammo may cause a failure to cycle and higher than standard pressure may promote excessive wear. The SR-556 gas operated piston system addresses both issues.



The Ruger SR-556 system utilizes a gas block with a barrel connecting orifice that permits gas to flow from the barrel to the block. However, instead of flowing directly into a tube and being carried to the bolt carrier, gas first flows through a metering body, a regulator, that offers four shooter selectable levels of flow. Labeled "Variable size gas ports" in the photo above, a smaller port is selected for higher pressure loads or to set the gun to single shot, manual cycle. Progressively larger ports are selected as ammunition load pressure diminishes. The parts are shown disassembled for the purpose of illustration as finger tip adjustments are made with the assembly installed, as pictured in inset. Maintenance of the Ruger assembly is easy to accomplish; the regular pin, #4 in the picture above, is pushed left to right to the detent position, the regulator is indexed to the gas block flat and the parts pull out with light finger pressure.



Another major feature of the SR-556 operating system is labeled #3 in the photo above, the piston that replaces the function of the AR standard gas tube. With the Ruger system, gas from the regulator acts on the piston and the piston drives a transfer rod that mechanically acts on the bolt carrier. Photo left illustrates the difference between a bolt carrier key to contain a gas tube versus the Ruger where a bushing driven by a transfer rod actuates the bolt carrier. The SR-556 system runs cooler during extended fire and there is no more powder residue passed to the bolt carrier through the gas tube.

After a lot of SR-556 shooting, both cartridges, with both factory and handloaded ammo, I've found the gun to be highly reliable, it does run a lot cooler in heavy use and it is easy to maintain. The latter not a small point as I am incredibly lazy and I do not take joy in cleaning firearms. I do it, but I don't like it. As a handloader, the adjustable gas bleed is greatly appreciated.

Control is where you find it... and where it is expected

If you are an AR shooter, your fingers and hand motion will find all of the controls as they have been conditioned to expect. Actuation of controls is crisp and positive; everything thunks, pops and slaps as they should. The finish did not rub off under controls at contact surfaces. The trigger pull is... more than casual, but who wants a hair trigger on an AR?



After some shooting time, the Ruger was easy to shoot with the stock pulled full out and easy to carry with the stock pushed in. The six adjustment lengths made it easy to adapt to clothing and method of carry. The forearm with rail covers in place made for a very comfortable hold. The metallic sights were a little low for me, just like its AR-15 brothers. With a scope on medium height mounts, eye alignment was very comfortable.

On to the real fun...



L-R, 5.56 NATO, 6.8 Remington SPC, the Real Guns 7mm-30 RG wildcat, and the 30 Remington AR. All of these work well with the AR platform. Higher pressure limits of single shot and bolt action firearms allow them to be even better.

The story here is the 6.8 Remington SPC in concert with an excellent SR-556. I encourage you to read the original coverage of the SR-556 as Part II is headed toward live fire and handload assessment. The 6.8 SPC is a long case in a caliber that typically requires long bullets. Consequently, bullets intended for this specific application, like the 110 and 120 grain Hornady and Remington bullets, have a short ogive to assure there is plenty of parallel shank to anchor the bullet in the case. A different approach is the short stubby case to allow for longer bullets in the same allowable 2.260" COL. The 0.277" bullet was selected by Special Forces saw this cartridge as the ideal balance of bullet weight and exterior ballistics and the stubbies are usually larger in caliber. So the results of Part II should be interesting

After handloading a hundred rounds of ammo, and shooting the SR-556 6.8 SPC for accuracy, data collection and handling, I ended up handloading a hundred rounds more as it is a tough gun to put down. If this gun has recoil, I wasn't able to find it, even though the 6.8 Remington SPC provides significantly more power than the 5.6x45mm. I found myself comfortably and accurately shooting the SR-556 with metallic, optical and FastFire II dot sighting systems. Even when shooting what is rapid fire to me, 3 shots in 2.7 seconds, it was easy to keep the gun on target. I can honestly say that the SR-556 is a... nifty looking rifle, but the best part of the SR-556/6.8 happens when it is being fired. The Ruger SR-556 is an excellent execution of an AR derivative design. Ruger includes the right accessories and the adjustable gas/piston operating system is a  feature contributes significantly to the firearm's performance; heat reduction, cleaner operation, greater reliability, and recoil dampening.




My specific interest in the 6.8 SPC version of the SR-556 is based on a preference for a cartridge of greater power than the standard 5.56x45mm. The more AR type guns are rebranded as modern sporting rifles, the more an alternative to the little 5.56 round is needed. i.e. If the cartridge won't get it done on medium size game in hunting applications, there isn't a great deal of value in extended magazines, Picatinny rails and rifle ergonomics made for military, police and security applications. With the 6.8 Remington SPC chamber, the SR-556 becomes much more than the Transformer like appeal of an AR 15. The hardware function and cartridge performance represent a perfect fit for many security and hunting applications.

A verbal regurgitation, but required for the sake of context. The 6.8 Remington SPC originated as a 2003 cooperative effort with contributions from Steve Holland and Cris Murray from the 5th Special Forces Group of the United States Army, Remington and Hornady.  The military spec'd the objective of enhance combat effectiveness of the M4 carbine, Remington designed a case based on a shortened version of their old 30 Remington that would work within M4 system parameters and Hornady produced a short ogive 115 grain bullet that would provide maximum performance with a 2.250" maximum COL. The SAAMI COL spec is 2.260", however, single shot and bolt action rifles are often loaded to a longer than specification 2.355" COL. Yes, how much clarity would you like in your mud?

Go big... but not too big

There have been lots of successful attempts at boosting the performance of a 2.260" cartridge without bumping pressure too high, increasing gas volume too much, or bumping bolt thrust. If a cartridge designer stays relatively close to home, it is relatively easy to end up with a reliably functioning AR. If the envelope is pushed too much, it's like teaching a giraffe how to roller-skate. Not that I know from experience, but I have been know to create two AR 7½ from one AR 15 when applying a bit too much... exuberance during cartridge development. The table below doe a pretty good job of illustrating differing philosophies in meeting a more power - short case goal.

Cartridge Bullet
Diameter
Inches Case
Diameter
Inches Case
Length
Inches Overall
Length
Inches Capacity
Grains Maximum
Pressure
PSI
5.56x45mm 0.224 0.377 1.760 2.260 28.5 62,366
6.5 Grendel 0.264 0.439 1.524 2.244 35.0 52,214
6.8 Rem SPC 0.277 0.422 1.687 2.260 34.0 58,740
7mm-30 RG 0.284 0.501 1.530 2.260 45.5 *55,000
30 Rem AR 0.308 0.501 1.530 2.260 44.0 55,000
450 Bushmaster 0.452 0.500 1.700 2.260 60.0 38,550
50 Beowulf 0.500 0.540 1.656 2.250 69.0 33,359
*AR Platform. 62,000 PSI for Bolt and Strong Single Action

As caliber increases, either cases shorten to accommodate relatively long bullets within the 2.260" COL envelope, or cases remain relatively long, however, pistol bullets are adapted for their short ogives and the resulting short COL. Some cartridges run at a relatively low pressure or incorporate a rebated rim. I thought the 30 Remington AR had about the best geometry and bullet form for an AR 15 type firearm, but I think the lighter weight bullets utilized with this round have too low sectional density and too low of a ballistic coefficient for long range shooting. If shooting was close in, it would seem either the 450 Bushmaster or 50 Beowulf would be a better choice.



The original AR 5.56 NATO, far left, followed by the 6.8 Remington SPC, the Real Guns 7mm-30 RG wildcat, and the 30 Remington AR. All of these work well in concert with an AR platform and the higher pressure limits of single shot and bolt action firearms allow them to be even better.

We published all of the 7mm-30 RG cartridge and tooling specifications, tooling sources and accumulated data on Wikipedia in an effort to share, but some Wiki-Weenie editor deleted the article because he felt the originating source should not be the author. I guess Wiki hearsay, Wiki misinformation and Wiki conjecture are essential to their formula. In any event, the full information appears here in RealGuns and it is free to access.

We played around on either side of 0.308" with the 30 Remington AR case and decided on a 7mm version. It worked well with the AR platform, but also worked quite well in bolt or single shot actions, operating at higher pressure. So we wanted to see what a 0.277" round would deliver.

A man with a plan. Theoretically speaking...

http://www.realguns.com/images/hndlded68spcamo.jpg

Left to right: GS Custom 110 grain HV, Sierra 90 grain Varminter, Hornady 110 grain flat base hollow point, and Hornady 130 grain soft point. Generally speaking, the shorter the bullet the better the potential performance as the 6.8 Remington SPC case is modest at best when it comes to capacity and long for this caliber, which generally has longish bullets. In fact, Hornady makes two bullets specifically for the SPC, a 110 grain and a 120 grain, both with a stubby ogive similar to their LEVERevolution bullets for lever guns. I did not load either of these as it didn't seem necessary for this effort. As a separate note, Hornady's current manual, 8th edition, lists the CCI 41 primer for this cartridge, which utilizes a small rifle primer when loading Hornady brass. Remington brass, as used within this project takes a large rifle primer..

The decision to use the long GS bullet was predicated on the bullet's bore diameter undersize shank and caliber size thin driving bands. The bullet offer very little resistance to forming when they contact rifling and, therefore, have very low start pressure. Subsequently, were able to run a bit stiffer charge behind these bullets than we could with a conventional bullet. The load data can be found on a dedicated load sheet, but the table below gives a pretty good idea of how we fared.
Bullet Type Bullet
Weight Muzzle
Velocity
FPS Muzzle
Energy Ft/Lbs Group
Inches
Sierra Varminter 90 2764 1527 0.9
Sierra Varminter 90 2810 1578 0.9
Sierra Varminter 90 2766 1528 0.6
Sierra Varminter 90 2774 1537 1.1
GS Custom HV 110 2403 1410 1.2
GS Custom HV 110 2444 1459 1.0
GS Custom HV 110 2461 1479 1.1
Hornady 110 2497 1523 0.9
Hornady 110 2482 1505 0.6
Hornady 110 2521 1552 1.1
Hornady 130 2313 1545 0.8
Hornady 130 2276 1496 1.1
Hornady 130 2393 1653 1.0
Speer SP 130 2370 1622 0.9
Speer SP 130 2293 1518 0.8
Speer SP 130 2403 1667 0.8






I shot all groups from a rest to minimize the effects of my highly evolved, but mediocre marksmanship out of the group size equation. I found that with adequate ear protection this was an easy gun to shoot eyes open.

There was nothing exceptional noted during either handload development or shooting sessions. All configurations fed and cycled reliably and there was no need to make adjustments to the gas block to accommodate safe or reliable function of any of the handloads. Mil Spec primers were not used, just conventional large rifle types, without incidence. Powder choice was a little restrictive, even utilizing powders recommended as they appeared in reference reloading manuals. I did try Alliant 2400 as a partial case charge as history has shown me pressure remains predictable. Not the case with H110 or Win 296, so these stayed home.

I don't think powder inhibited the gun's or the cartridge's potential. A couple of tenths grains would not turn this cartridge into a 400 yard deer or varmint rifle, nor would it diminish the round's performance enough to make it a sub 250 yard performer. I think killing power in regard to deer or hogs is better than a 30-30 WCF which means the SR-556/6.8 has a good deal more utility than other AR 15s chambered for the 223 Remington... if you'll forgive my lack of metricality. Yes, I did just invented that word. Thank you for noticing.

It was like hanging out with the cool kids... I would imagine



I had to work really hard to bring this project to a conclusion. I was shooting off lots of ammo, swapping sight systems trying different loads and shooting from a close seven yards to a couple hundred. I meant to snap in the rail guards, but I never noticed them missing and the rails gave me a nice grip on the gun. When I was done spraying brass and powder residue all over the gun, and picking up empties nine or ten feet away, I just wiped it off with my special cleaning kit, an old sweat sock sprayed with gun oil and it looked as good as it did when I took it out of the box... the rifle, not the sock. I spent about half an hour shooting rapid twenty five yard groups with metallic sights, over a rest until one grouped under two inches and I could quit while I was ahead. Amazing how much concentration a person can muster when they aren't being banged around by muzzle blast.

If I were going to change anything, it would be... nothing. I would like to see the system evolve with maybe a short target weight barrel with no flash hider and chambered for the 7mm-30 RG. Or maybe just a handful of uppers is a variety of chambers. Beautiful gun. Ruger did an excellent job with the SR-556 and 6.8.