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Ultimate Gas Impingement AR: KAC SR-15E3 SBR Review

by Peter G. Kokalis   |  December 10th, 2013 3

Knight’s Armament Company (KAC) is well-known for the excellent 7.62x51mm NATO M110 Sniper Weapon System they supply to the United States Armed Forces. Their highly regarded sound suppressors and night vision equipment are equally acclaimed throughout the special operations community.

Not so well known is the fact that they also manufacture what is arguably the most technologically advanced series of 5.56x45mm NATO, direct-impingement gas-operated, rifles in the world. Although all are based upon the late Eugene Stoner’s AR-15 design, the innovations found on the KAC SR-15 (“SR” obviously standing for Stoner Rifle) series are nothing short of impressive, and significantly beyond anything encountered in competing product lines using this system of operation.

I assembled the rifle used in this test and evaluation in the following manner. I first purchased a KAC (Product Number 30355) SR-15E3 SBR (Short Barrel Rifle) Carbine upper receiver assembly with a 14.5-inch dimpled barrel (manufacturer’s suggested retail price $1,875) to which I then permanently attached a KAC MAMS Muzzle Brake by pinning and welding to make it BATFE-compliant with a barrel length of 16 inches.

After that, I purchased a (KAC Product Number 25780) SR-15 IWS lower receiver assembly (manufacturer’s suggested retail price $750). The result was a semiautomatic-only KAC SR-15E3 IWS SBR Carbine with a 16-inch barrel.

But before we examine this unique weapon with closer scrutiny, let’s take a look at the history of what has become one of the most important military small arms manufacturers in the United States.

More than 30 years ago, C. Reed Knight, Jr., founded Knight’s Armament Company as a research and development facility. It has subsequently evolved into a truly premier manufacturing operation that offers complete weapons systems, sound suppression devices, night vision electro-optics and modular accessories for small arms. KAC is justifiably recognized for advanced developments in small arms designs, components and accessories, with research and development, manufacturing, assembly and prototype/product testing all performed at the current facility in Titusville, Fla.

In 1982, Knight founded KAC in Vero Beach, Florida to focus on products that met the very specialized requirements of the Special Forces community. KAC is currently one of the largest small arms manufacturers in the United States.

With the active collaboration of the late Eugene Stoner, KAC has designed some of the most innovative modern small arms systems ever conceived. From Reed Knight’s first endeavors in sound suppression, to his collaborations that produced a number of weapons systems including the SR-15 series, the development of KAC’s patented rail system and night vision electro-optics, Knight’s Armament Company has consistently pioneered the manufacture of innovative and trend-setting small arms and accessories.

The first landmark products to originate at KAC came during the 1984 to 1989 time frame with the introduction of the 9mm snap-on sound suppressor, the first “LEGO kits,” which included the forerunner of the RIS/RAS and URX rail systems, and with input from Eugene Stoner, the development of the All American 2000 pistol (sold to Colt) and the SR-25 precision rifle platform.

During the early 1990s, when Eugene Stoner officially joined the KAC team, the first tests of Knight Vision electro-optics commenced. The adoption of the M4 Carbine occurred in 1994, together with USSOCOM’s adoption of Knight’s Rail Interface System (RIS) and the first SR25 rifle contract by the U.S. military.

The new millennium saw the US Navy’s adoption of the Mk11 Mod0 (their designation of the SR-25), followed in 2002 by the transfer of KAC’s facilities from Vero Beach to a 450,000 square feet building complex in Titusville, Fla.

Between 2003 and 2005, several of KAC’s most impressive products were introduced, commencing with the Universal Rail Extending (URX) free-floating rail system. This was followed shortly thereafter by the U.S. Army’s acquisition of Knight Vision’s UNS (Universal Night Scope), and the subsequent adoption of the M110 SASS from the XM110 program, which was awarded to KAC in 2004.

Knight’s maintained this pace of high quality innovation by releasing the PDW (Personal Defense Weapon) in 2006. The PDW platform was built around a proprietary 6x35mm cartridge, developed in-house for specific applications within the special operations arena.

In recent years, KAC has continued its mastery of quality innovation with products such as their “Ball Mill Lightened,” or “Dimpled” barrels, Carbon Cutter Bolt Carriers, the Pressure Reduction System, Integrated Sights, and Ambidextrous Controls.

Products like their Micro Iron Sights, 45° Offset Iron Sights and Optics Mounts, and slick-sided URX III and URX 3.1 continue the KAC tradition of effective and innovative products designed around the real-world needs of professional end users.

The year 2012 saw the introduction of eight new weapons for the commercial and law enforcement markets, divided by caliber and application. These new weapons are equipped with the latest KAC technology, backed by decades of battlefield experience.

KAC SR-15E3 IWS SBR Carbine
The 5.56x45mm NATO SR-15E3 IWS SBR Carbine used in SGN’s test and evaluation is equipped with several of KAC’s latest proprietary features. The major features of this rifle include the URX 3.1 rail system, a hammer-forged, chrome-lined barrel, the innovative E3 bolt group, ambidextrous controls, a two-stage match-grade trigger mechanism and the optional MAMS muzzle brake.

As described above, the barrel’s non-NFA overall barrel length of 16 inches (406mm) was in this case achieved by permanently pinning the optional MAMS muzzle brake onto a 14.5-inch barrel. When supplied with a full-length 16-inch barrel and M16A2 flash suppressor, the barrel length becomes 17 inches (432mm).

The dimpled medium profile barrel has six grooves with a 1:7 right-hand twist and is thus best utilized with the heaviest 5.56mm projectiles. The rifle’s overall length is 33 inches (838mm) with the four-position, retractable stock fully collapsed and 36 inches (914mm) with the stock fully extended. The weight, empty, is only 6.6 pounds (3kg).

The rifle furnished to SGN has a hard coat black anodized finish, but dark earth is available as an option at slightly greater cost. KAC’s URX 3.1 rail system is a lightweight, free-floating, continuous-top rail system with MIL-STD-1913 rail interfaces.

It was specifically designed to provide operators with the most diverse number of placement options for accessories possible. To relieve all pressure, contact and stress from the barrel that might adversely affect the barrel’s harmonics, the URX system was designed to be free-floating. Except at the point of attachment to the receiver, the barrel is not in contact with the rail system. As a consequence, accessories mounted on the URX or their manipulation cannot affect the barrel’s accuracy potential.

The URX 3.1 rail system features a flat, continuous rail at 12 o’clock, several inches of rails at the forward and aft of the 3, 6 and 9 o’clock locations, and slick-sided areas in the middle sections (except where specialized, textured hand gripping panels are attached). By eliminating areas of the rail that are commonly unused, the slide sides offer a slimmer diameter for gripping, as well as an overall reduction in weight.

The integrated hand stop panel designed for attachment to the URX rail system at the 6 o’clock position was designed to accommodate the extended support handgrip now so frequently employed by tactical shooters and many special operations military personnel.

This gripping method involves placing the support hand at the far forward end of the rail system with the index finger extended and with the other fingers wrapped around the rail system. There is, in addition, a rubbery, 1.75-inch thumb stop that snaps onto the front end of the 12 o’clock MIL-STD-1913 rail to further enhance this type of firing grip.

With the elbows tucked in and the support arm completely extended, this firing position creates an isosceles triangle, which supposedly slightly mitigates the recoil impulse, increases the speed of lateral movement when necessary and improving overall control in CQB scenarios.

A number of special operations shooters, and others well known in tactical circles, such as Kyle Defoor, Jason Falla, Larry Vickers and Kyle Lamb, have jumped on this bandwagon. However, the jury is still out on this technique and many, including myself, disagree.

But I intend to continue experimenting with this technique using the KAC SR-15E3 IWS SBR Carbine as a platform. The KAC Deluxe Panel Kit (KAC Product Number 30409-BLK) sells for $85.94 and allows you to set up the SR-15E3 IWS SBR Carbine for either the isosceles shooting grip or a more conventional configuration.

KAC has removed the integrated, backup iron sights found at the 12 o’clock position on earlier URX rail systems. Many operators were almost entirely utilizing optics only. To resolve this issue, KAC developed its now famous 45° Offset Backup Iron Sights (BUIS).

KAC’s 45° Offset Rail Mounted Micro Folding Sights provide operators with an important advantage in rapid target acquisition over a 12 o’clock BUIS system. In addition, they are instantly available if the optic sight fails. They are of the same type as KAC’s fully adjustable Micro Flip Sights designed for 12 o’clock mounting, but are attached to a low-profile arm that interfaces with MIL-STD-1913 rail systems and cants the sights at a 45° angle.

Both the front and rear sights fold down to a low-profile configuration that in conjunction with the arm that rests in the rail recesses, provides an exceptionally lightweight and unobtrusive BUIS system. The sights provide the operator with an entirely different line-of-sight than the optical sight mounted at 12 o’clock.

As a consequence, just a quick twist of the rifle permits instant access to them. They’re especially valuable for rifles equipped with medium to long-range optical sights, as they provide the instant capability to engage unexpected close-range targets. If the optical sight fails, they can be used to engage a target immediately without removal of the optical sight.

The rear sight is a large aperture type adjustable for elevation in 100-meter increments from 200 to 600 meters. A knob on the sight’s right side permits windage adjustments. The front sight is a thin post with protective ears than can be adjusted for elevation zero. This excellent optional accessory sells for $281.25.

KAC uses a hammer-forging process on their 5.56x45mm barrels and chambers, which increases the quality of the interior finish, in addition to increasing the durability and life of the barrel. After the interior of a drilled barrel has been honed to a fine, clean finish, the barrel is placed over a mandrel and hammered into form.

The barrels are subsequently heat-treated to relieve the stress induced during hammer-forging. The barrels and chambers are chrome-lined after hammer-forging both to increase barrel life and resistance to fouling and corrosion. Chrome is approximately twice as hard as standard 4150 CMV steel and also impedes the lead and powder fouling that bare steel can sometimes absorb into its pores.

Chrome lining protects the barrel from the high temperatures encountered when sustained full-auto bursts are employed and thus substantially increases barrel life while maintaining the barrel’s accuracy potential and the entire system’s durability and reliability.

KAC has pioneered an exceptionally innovative and effective means for reducing both weight and stress, while at the same time dissipating heat in its barrels. These so-called “Ball Mill Lightened Barrels” use a pattern of dimpling (rows of offset spheres) to increase the surface area and significantly enhance cooling over that achieved by either a standard or fluted barrel.

The stress inherently created by this dimpling process is relieved by adjacent rows of dimpling. From an engineering perspective, any barrel with this type of alteration will produce a yaw deviation when fired. However, KAC’s dimpling pattern substantially reduces this undesired side effect, reducing the point-of-impact shift through a thermal cycle.

Attached to the barrel by both pinning and welding is the new KAC MAMS (Multi-Axis Muzzle Stability) Brake, which came out of recoil management studies, conducted in cooperation with the U.S. Government. The MAMS Brake reduces the perceived impulse recoil by up to 60% and constitutes a truly neutral bias system.

The MAMS Brake is designed to diffuse high-pressure muzzle gases, which serves to increase the rifle’s accuracy potential and controllability, both extremely important in CQB scenarios. The MAMS has a pin projecting at 12 o’clock at its rear for indexing with and securing KAC’s QDC sound suppressors. The fins extending from the front of the MAMS Brake reduce the first round flash signature by up to 99% when compared to conventional types. In summary, the MAMS Brake increases weapon controllability, reduces the flash signature and perceived recoil impulse, while providing a neutral bias with sound suppressor compatibility. At the time of this writing a price for the optional MAMS Brake has not yet been decided.

All of KAC’s rifles come equipped with a two-stage, match-grade trigger system with a guaranteed 4.5-pound trigger pull weight. The trigger pull weight on the rifle sent to us for test and evaluation was a very clean 4.25 pounds. During the first stage of the trigger pull, very little resistance is felt. When the second stage is in encountered, a maximum pull of no more than 4.5 pounds is required for let-off. Two-stage triggers are a tradition associated with America’s history as a nation of riflemen. The trigger guard can be rotated down for use with thick arctic gloves by removal of a small roll pin.
The controls on KAC rifles are ambidextrous. That is, truly ambidextrous as many manufacturers claim ambidextrous controls, but in reality they are not. The SR-15E3 IWS Mod 1 Carbine’s selector lever and magazine catch/release on each side are mirror images.

The bolt release levers are identical, but placed vertically on the left side and horizontally on the right side. The selector lever on the right side is scalloped to provide a lower profile on the usually non-operating side of the receiver to prevent interference between the lever and the operator’s glove or hand.

One of the KAC SR-15E3 IWS SBR Carbine’s most important and unique features is the E3 Enhanced Bolt. This proprietary component was specifically designed for SR 15 and 16 models. Its very special features include an enlarged lug pattern, rounded lugs, reduced cam pin (to accommodate the thicker web around the cam pin hole), dual-spring extractor and unique firing pin design.

The KAC E3 Enhanced Bolt has an enlarged lug pattern that provides more positive engagement when in battery and its rounded lugs minimize chipping and cracking. This strengthened design increases bolt life and ensures correct seating in the battery position.

The E3 bolt’s extractor achieves total reliability in an area that has plagued the M16 system since its inception. Shaped like a rounded “T” and sometimes referred to as a “butterfly” configuration, the extractor’s pivot point has been shifted forward to better balance the extractor and apply constant pressure on the cartridge case’s extractor groove to substantially reduce the once common failure-to-extract malfunction.

Another unique feature is the use of two full-size extractor springs instead of the conventional micro spring. This not only reduces spring compression while the extractor is in motion, but provides a failsafe if one spring loses compression strength, as the rifle will continue to function normally.

The E3 Enhanced Bolt has a thinner cam pin and a firing pin of reduced overall size. The thinner cam pin permits a thicker bolt web, which is a frequent point of failure on AR bolts. The scaled-down firing pin was designed to accommodate the thinner cam pin.

The full potential of the KAC E3 Enhanced Bolt is especially realized during sustained full-auto firing sequences. Its enhancements result in significantly increased durability and service life. This is quite simply the best AR bolt available.

The SR-15E3 IWS SBR Carbine comes equipped with a black, four-position Lewis Machine & Tool (LMT) SOPMOD buttstock. The SOPMOD buttstock was originally developed by NSWC-Crane for the M4A1 carbine.

This stock features sloped sides for an improved cheek weld, as well as waterproof battery storage compartments and a thick, checkered, synthetic rubber butt pad. I don’t carry batteries in buttstocks of this type, as they only serve to further increase the weight of already overloaded rifle systems.

But there’s no doubt that these battery compartments widen the top of the stock and significantly improve the cheek weld. The SR-15E3 IWS SBR Carbine also comes equipped with an LMT forged charging handle with tactical latch.

The M16A2-type black polymer pistol grip has deep longitudinal grooves along its rear face. There’s a finger swell 1 inch below the trigger guard on the grip’s front face and an anti-slip protrusion at the base of the grip.

For insertion into the SR-15E3’s flared magazine well, a 30-round, two-position-feed, staggered-column GI magazine manufactured by Eddie Okay Industries, Inc. in New Britain, Connecticut was included. However, KAC’s white polymer, anti-tilt follower was installed. A magazine safety insert was also provided.

Two pushbutton (aka QD or snap-in) sling swivel attachments came with the SR-15E3 and there are ambidextrous holes to snap them into on the SOPMOD buttstock and at three places on each side of the upper and lower receivers.

Our test and evaluation of the KAC SR-15E3 IWS SBR Carbine provided us with an opportunity to use an interesting new tool for Direct Gas Impingement M16/AR-15-type rifles. The CRT-15 Carbon Removal tool is manufactured by the Magna-Matic Defense division of Magna-Matic Corporation. Zinc-plated, the CRT-15 not only removes carbon fouling from the bolt carrier but from the tail of the bolt as well. This useful armorer’s tool sells for $39.99.

Because of the cost of the commercial version of KAC’s M110 sniper weapon system, some believe that KAC’s law enforcement and civilian semiautomatic-only rifles and carbines are expensive. KAC’s manufacturing facility was established to meet the U.S. Government’s rigid standards for the production of MilSpec products used by our nation’s war fighters. All of KAC’s weapons are manufactured using the same standards, which include almost unending inspections and stringent quality control specifications of the highest possible level.

The SR-15E3 IWS SBR Carbine used in the SHOTGUN NEWS test and evaluation was built to the same standards as the U.S. military’s M110 sniper weapon system. When you add up features such as the URX 3.1 rail system, a hammer forged, chrome-lined, Ball Mill Lightened barrel, the innovative E3 bolt group, ambidextrous controls, a two-stage match-grade trigger mechanism, LMT SOPMOD buttstock and tactical latch retracting handle and the optional MAMS muzzle brake, the manufacturer’s suggested total retail price of $2,625 is actually more than reasonable.

This is the finest quality, most exceptional direct gas impingement AR-15-type rifle I have ever tested, bar none—really hot ticket and absolutely recommended without reservations of any kind.

Optics for the KAC SR-15E3 IWS SBR Carbine
Selecting an optical sight for a caliber 5.56x45mm NATO tactical rifle of this type presents a dilemma of no small consequence. For actual tactical deployment in an urban operational area with a relatively short-barreled rifle, a scope with a magnification range of 1X to no more than 4X is clearly indicated. However, it’s impossible to determine the rifle’s real accuracy potential with magnification that low. As a result, we selected two scopes, one for tactical deployment and one to test the rifle’s accuracy potential.

Leupold & Stevens, Inc. has been making riflescopes for 102 years. Their product line includes optical sights that cover hunting, target shooting and tactical applications. They sell more tactical scopes to the U.S. government than all other scope manufacturers combined.

There are several reasons for that. Most of their rifle scopes, and every component thereof, are made entirely in the United States. Equally important, the quality and optical features of their scopes are outstanding. I selected the Leupold Mark 4 8.5-25x50mm LR/T M5 Illuminated Reticle Riflescope with their relatively new and innovative Tactical Milling Reticle.

Overall length of this scope is only 14.4 inches (365.76mm) with a weight of just 22.5 ounces (638 grams), which is quite incredible for a tactical scope with a three times magnification range from 8.5X to 25X. The tube diameter of the scope is 30mm. A 30mm tube with its thicker walls has considerably more cross-sectional area inside the tube than most 1-inch tubes with their thinner walls. Once this additional area is available, the erector tube inside the scope body (which carries all lenses except the ocular and objective lenses) and its lenses can be increased in size to transmit more light and thus yield greater resolution and a brighter image. Furthermore, a heavy 30mm housing is more shock-resistant than any 1-inch tube.

The eye relief, an extremely important factor in assessing the operator’s ability to quickly come on target is 3.7 to 5.3 inches, which is really excellent, as high magnification scopes all too often have such a narrow range of eye relief that it’s difficult to acquire the target at the highest power settings.

The field of view at 100 yards is 4.4 feet at 25X magnification and 11.2 yards at 8.5X magnification. Leupold’s Xtended Twilight Lens System optimizes the transmission of low-light wavelengths, so you can see the details of low-light scenes in greater, brighter detail than most other scopes. The image is sharp and brilliant.

In my opinion, this is one of Leupold’s finest tactical scopes with an ideal magnification range for battlefield applications at longer ranges. The manufacturer’s suggested retail price is $1,999.99 with the Tactical Milling Reticle pattern. Leupold 30mm, aluminum alloy, high rings (#57291) cost $224 and their really great aluminum alloy, flip-up lens caps sell for $132. I prefer totally opaque lens covers because they force the operator to flip them up, as the see-through types always degrade the scope’s optical qualities.

Scopes in Leupold M5 series have both windage and elevation adjustment knobs that feature audible and tactile feedback from half-minute and one-minute numbered divisions with 0.1-minute MIL-click-stops clearly marked between each one-minute division. The ability to make one-tenth-minute adjustments is an important attribute. Total elevation travel on this scope is 75 moa, and the windage adjustment is 70 minutes and each complete revolution raises or lowers the point of impact by 5 minutes.

The elevation knob also has a horizontal scale that is used to keep track of the number of revolutions that the dial has been turned. In addition, there is a built-in anti-backlash system that guarantees repeatable accuracy from click to click, and back again.

Most snipers use an elevation adjustment system such as this by zeroing the rifle and scope at specific ranges and writing the elevation adjustment settings on a range card attached to the rifle’s buttstock. The setting to which the elevation adjustment knob must be rotated for a specific distance is usually referred to as a “come up” by those who move in this elite loop. Leupold’s Mark 4 M3 scopes have half-minute adjustment increments. This is too coarse, in my opinion, for really long range shooting. Remember, 1 moa is the equivalent of 10 inches at 1,000 yards.

This Leupold Mark 4 8.5-25x50mm LR/T M5 Illuminated Reticle Riflescope is equipped with their new Illuminated Tactical Milling Reticle (TMR) pattern. Mil-dots were developed by the USMC in the late 1970s to assist Marine Corps snipers in estimating distances. It is now the standard reticle pattern with all branches of the US Armed Forces. The term “mil-dot” comes from “mil”—a unit of angular measurement used in artillery and machine gunnery and equal to 1/6400 of a complete revolution—and the fact that the dots are spaced in 1 mil increments on the crosshairs. It should be made clear that the dots (or lines) themselves are not measured in mil increments, but rather in increments of MOA.

The principle behind Leupold’s TMR is to expand on existing mil dot reticle designs by offering users more ranging tools in the form of various sized and various spaced aiming points on the horizontal and vertical stadia lines. This permits greater ranging and shooting precision than all previous range estimating reticle systems. The TMR reticle subtends exactly like all existing mil dot reticles and variations thereof, but with greater accuracy. Aside from mil hash marks, the TMR reticle offers areas of 0.2-mil subdivisions to precisely measure the common one-meter target height from 500 to 1,000 meters and beyond. This has previously been the most difficult task in long-range shooting, since this entire range lies in the span between one and two mils. The positions of the 0.2-mil subdivisions are intentionally placed on the periphery of the fine crosshair in order to keep the central area clutter-free. All existing mil dot calculations and formula tools are compatible with the Leupold TMR system.

The Leupold Mark 4 8.5-25x50mm LR/T M5 Illuminated Reticle Riflescope was attached to the 12 o’clock MIL-STD-1913 rail interface of the KAC SR-15E3 IWS SBR Carbine by means of a GG&G Accucam Quick-Detach MIL-STD-1913 Scout Rail. This quick-detach rail raises the line of sight by a half-inch. Designed for a wide variety of optical sights, its integrated Accucam Quick-Detach lever system provides the operator the option to quickly utilize other optical devices. Its rail is machined from solid billet 6061 T6 aluminum alloy with a Type III hard coat anodized matte black MilSpec finish, its tension on the rifle’s rail can be adjusted and it provides zero repeatability of 1/2 MOA. The Accucam mechanism itself is machined from solid billet 4140 steel and manganese-phosphated matte black to MilSpec. The price is $158.95.

The Leupold Mark 4 8.5-25x50mm LR/T M5 Illuminated Reticle Riflescope is a great piece of glass, but too much magnification for breaking down doors and dynamic entries in the barrio with a 5.56x45mm tactical rifle equipped with a relatively short barrel. So, after the accuracy portion of our test and evaluation, we installed a Leupold Mark 4 1-3x14mm Close Quarter/Tactical (CQ/T) Scope, which was specifically designed for rifles in the M16/AR-15 series and combines the strengths of a red dot sight and variable-power scope. The clear, bright Leupold Circle Dot illuminated reticle provides an outstanding aiming point for the operator, with instant target acquisition in close, urban combat environments. There are ten illumination settings, including two compatible with night vision devices, without overwhelming the operator’s natural, low-light vision. One AA battery powers the unit for up to 600+ hours at the medium setting. At 1X, the CQ/T Scope functions as a non-
magnifying, illuminated sight for tactical applications at extremely close ranges. Targets at medium ranges can be successfully engaged when the scope is powered up to 2X or 3X. Butler Creek flip-up lens covers, the Mark 4 CQ/T flat top mounting bracket, and a carry handle mounting stud are all included.

The elevation and windage adjustment markings include 0.5 MOA clicks. The linear field of view at 100 meters is 37.5 meters at high magnification and 13.9 meters at low magnification. The weight is only 17.5 ounces (496 grams), with an overall length of 8.8 inches (22.4 cm). The all important eye relief is 2.8 to 2 inches (71 to51mm). Both the elevation and windage adjustments have a range of 90 MOA. The manufacturer’s suggested retail price for the Leupold Mark 4 CQ/T scope is $1,124.99, but it typically sells for about $900. It is being fielded by special operations personnel in Afghanistan. This is a superb match with the KAC SR-15E3 IWS SBR Carbine.

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