Colt Defense’s introduction of piston operated M16/M4 type rifles over the past 3 years has been customer driven and expands the M16/M4 family of weapons. Prior to that, they had experimented with a piston driven mechanism in the late 1960s known as the Colt model 703. This piston system was looked at by the U.S. Army and they came to the conclusion that there was no benefit of the piston system over the direct gas system and the program was dropped.

In the last three years, Colt Defense has been involved in two military programs and one law enforcement program dealing with piston operated firearms. The first was the Special Operation SCAR (Special operations forces Combat Assault Rifle) and they submitted three rifles. The Type A utilized their proprietary one-piece upper receiver and a direct gas operating system. The Type B was a standard M4 upper receiver with an A.R.M., Inc. SIR (Selective Integrated Rail) system with a direct gas operating system. The Type C SCAR entry was Colt’s piston operated carbine utilizing their proprietary onepiece upper receiver. The Type C SCAR, as of this writing, is the only M16/M4 based weapon to ever compete and finish an official U.S. government test and evaluation program. The Type C would later be updated and improved, which leads to Colt’s current LE1020 weapon system. The second military program Colt’s piston system would be used in was the OICW (Offensive Individual Combat Weapon). However, the program was cancelled before Colt could complete the design of that family of weapons.

While Colt intends to offer only one piston operated weapon in its catalog, they are currently integrating the best of both designs into their final offering. The first is known as the LE1020 and the other is what Colt calls their M5. In this article we will get a first-hand look at the M5 carbine. This carbine has been shown at some industry trade shows but never before in its entirety. Small Arms Review gets the exclusive first look at Colt’s other piston carbine: the M5.

The M5 Carbine

The M5 uses a conventional short-stroke tappet piston where a two piece piston/ operating rod strikes the face of the bolt carrier group driving it rearward and the piston rod returns to its forward position by a spring. However, unlike the Colt LE1020, the M5 uses a conventional M4 flat top upper receiver cut with extended feed ramps instead of Colt’s proprietary one-piece upper receiver. The piston itself rotates and locks onto the end of the operating rod. There is a spring and a stop on the rear of the operating rod. The operating rod/piston is inserted from the front of the receiver, pushed inward and guided into the front sight base. In Colt engineering testing, the M5 has fired 18,000 rounds without cleaning (other than the bore of the rifle) and without malfunction.

There are two current front sight base designs for the M5. The first is the one we just spoke of and is primarily for use with the standard upper receiver. The second more so resembles that of the LE1020 where the piston assembly is inserted from the front of the front sight base, slid back into position and held in place by a detent pin. This is designed for use with the one-piece upper receiver.

The front base is also very different from the LE1020 with a change to the front sight itself. The LE1020 uses a front sight assembly that is identical to the standard M16/M4. The M5 front sight is more similar to an AKM. The same front sight post as the M4 is used but the wings that protect the front sight post are round in profile giving greater field of view in-between the front sight base’s sides and the front sight post. This was based on the project leader speaking with many different operators and getting their opinion on what they thought would improve the sight picture of the rifle.

The barrel itself has had some redesign work on it as well though still utilizing the standard M4 feed ramps on the barrel extension. There are modifications of the chamber area of the barrel and the changes further ventilate gas and helps with air circulation by drawing heat out of the receiver and out the handguards. The cycling of the weapon acts as an air pump. Colt engineers have made the M5 in 10.5- and 14.5- inch barrel lengths.

The current prototype M5 utilizes a standard double heat shielded M4 handguard on the bottom and a modified one on the top. However, Colt is currently working on a universal handguard that will include quad Mil-Std-1913 rails. Prototype hand guards have been manufactured using a steel handguard with aluminum heat shields. Final handguard material is still under evaluation.

The bolt carrier is very unique and is quite different from the LE1020 in the way that the carrier key is held in place to cope with being struck by the piston rod. The carrier key is locked in place in the bolt carrier by a T-slot. The male connector is on the carrier key and the female connector is the bolt carrier. Then a screw is torqued to specification and crimped into place. The head of the screw has heavy serrations and, when crimped, is held firmly in place. The inside of the top of the bolt carrier has to be machined out so the two-piece carrier key assembly can be put in place. Due to this being a piston operated system; the gas rings on the bolt are not needed. The bolt is the standard M4 bolt with the heavy extractor spring and black stiffer buffer.

The lower receiver is the standard M4 lower receiver. Part of the design of the M5 upper receiver was so that it could be placed on an existing M4 lower receiver using the standard H buffer. Colt has made a very conscious effort to keep interchangeability of the lower receiver in the event the U.S. military decides they want a piston-operated rifle. Colt would be able to offer a simple retrofit upper receiver group that will meet this potential requirement.

Side Folding Stock Assembly

Another project in development with Colt Defense is the design of a new sidefolding stock. This is accomplished by modifying the bolt carrier and the buffer extension is made significantly shorter. The bolt carrier has been heavily modified and does the job as the bolt carrier and the buffer. The rear of the bolt carrier has been modified so the recoil spring attaches to it. A bumper is placed on the rear of the bolt carrier. In order to assemble the modified lower receiver, the lower receiver is slid into position from the front and the takedown and pivot pins are engaged. To disassemble, both pins must be disengaged and the upper receiver slid out the front of the lower receiver. The stock pivots to the left side and the stock can be moved upward. When folded, the controls (i.e., selector, bolt catch) are easily accessible by the shooter. The stock is telescopic like the M4 and has four positions. The stock resembles that of the M4 as well but strengthened for the configuration it is in. The computer images provided by the Colt Defense engineering department show what they are working on. Models have been made and are currently going through testing.

Conclusions

The best features from both the LE1020 and the M5 will be combined into one. Colt has taken an incremental approach to their final release. Their piston designs have been proven in both the SOCOM trials as well as independent military testing in some foreign countries and it will be very interesting to see the final design. Nevertheless, the question remains as to whether the piston system will be the future of the M16/M4 weapon system, or will it be a passing fad. The debate continues and only time will tell.

Ever since the change over from the 7.62x51mm to the current 5.56x45mm there has been controversy between two fundamental schools of thought on what a military cartridge should be. The United States military can be summed up in one word: tradition. In the 1960s, that tradition clouded the vision that a new dawn of small arms development in both concept, mechanics and ammunition had come. Still entrenched in the “one shot, one kill” mentality, our soldiers were sent into combat in Vietnam with the M14 which many considered to be an inferior weapon to the AK47 assault rifle that the enemy used. The enemy were able to lay controlled large volume of automatic fire on positions which, in a meeting engagement, will win you a fight. Our soldiers were equipped with a heavy recoil rifle that was impossible to control on fully-automatic. The accuracy was of no use because the enemy could not be seen.

With much trepidation, the military adopted the AR-15 rifle destined to become the M16/M16A1. A new age of small caliber high velocity cartridges gave way to providing an individual soldier with firepower he had never had before. The 5.56x45mm cartridge utilizing a .224 diameter 55-grain full metal jacketed bullet proved to be a great asset to the American fighting man with the ability to have a lightweight rifle that was completely controllable on fully-automatic fire and the soldier could carry more than twice the combat load with an M16 than he could with the heavy M14 rifle.

The Third Generation M16A2 and M855 Ball

With the product improvements of the M16A2 the weapon system was enhanced. Due to the heavier 62-grain bullet, 1 turn in 7 inch rifling twist and new fully adjustable rear sight, both penetration and long range accuracy were enhanced. However, the culprit of the future problems with ammunition terminal performance would come from the ammunition, the M855 Ball.

The M855 Ball is a three piece bullet consisting of a copper jacket, lead plug and a steel penetrator core in the tip. This bullet was designed for a machine gun, not a rifle. With the onset of the Global War on Terrorism, terminal deficiencies were found using M855 Ball, particularly in M4 carbines, when striking thin, malnourished, Taliban and Iraqi soldiers. There was a serious inconsistency in at what point the bullet would yaw and splinter. Some lots of ammunition would penetrate only slightly within inches and yaw and splinter. Others would go straight through and never yaw nor splinter. This caused serous knockdown problems particularly with the decreased velocity of 14.5 inch carbine barrel.

A solution put forth by the United States Special Operations Command (SOCOM) was the creation of a new and improved 5.56x45mm cartridge that would increase long range accuracy and terminal performance with consistency. Accuracy is seriously degraded in the M16A2/A4/M4 due to the inherent characteristics of the M855 bullet. The penetrator core degrades accuracy and if not manufactured properly, the center of gravity will be off causing the bullet to become a “flyer.” Additionally, in the area of terminal performance, there was no consistency from lot to lot on what exactly it would do upon hitting a human target.

The answer was the Mk 262 MOD 1 cartridge that SOCOM adopted. This is a Sierra 77-grain open tip match bullet loaded by Black Hills. This is undoubtedly the most effective 5.56x45mm military cartridge in the world manufactured to match grade specifications with combat reliability. The long range accuracy was drastically increased and its terminal performance enhanced. This new projectile works well regardless of the type of target and performs considerably better on human targets in all known distances and conditions. This round is used almost exclusively by SOCOM operators in their M4A1 carbines and Mk12 rifles. This solution satisfied most of the SOCOM operators: except for one.

The 6.8x43mm REM SPC

The 5th Special Forces, particularly MSG Steve Holland, felt that there was still room for improvement. The concept was not original. The U.S. military experimented with the possibility of the 6mm caliber projectile being the ideal compromise of accuracy, range and terminal performance. In the late 1970s the concept was abandoned in favor of the 5.56x45mm battle rifle.

With the assistance of the U.S. Army Marksmanship Unit, Holland came up with a concept for a 6mm cartridge that could be fired in the M4 platform. The starting point was the .30 Remington cartridge which is merely a rimless .30-30 Win. round. It was trimmed and resized to accept a 6.8mm projectile. This concept, along with the hand-loaded experimental ammunition and CAD drawings went to Remington Arms. Remington wanted to aid in the Global War on Terrorism so they took on the task of developing the ammunition.

The 6.8mm REM SPC cartridge has been mostly a myth for the last few years due to the fact it has been developed quietly and kept proprietary. No ammunition has been made as of this writing in large quantities due to the fact the ammunition is being perfected before it will be shipped. Remington has reported three small runs of ammunition including ball and match grade ammunition. Hornady has also produced some loads in this new caliber.

The Firearms

Prototype rifles were manufactured by Precision Reflex, Inc. (PRI). Dave Dunlap assembled upper receivers with free floating handguards or A.R.M.S., Inc. SIR systems and the OPSINC silencer and muzzle break. Dunlap also was charged with designing the magazine which is different from the standard 5.56x45mm magazine. Currently, PRI has been the only producer of 6.8x43mm Rem SPC magazines. PRI also produces complete upper receivers chambered in the 6.8x43mm Rem SPC cartridge.

The Barrett M648

At the 2004 SHOT Show, Ronnie Barrett unveiled a new product line: his M648. This company, dedicated to .50 BMG caliber specialized weapons, introduced the first production 6.8x43mm Rem SPC firearm and has taken the lead on this project. While the cartridge is still technically under development (to be more correct would be to say “tweaked”), Barrett has teamed up with Peter Forras to work with Remington to optimize the weapon and cartridge. Forras has spent much time down at Remington working with them to perfect the 6.8x43mm cartridge.

The rifle has been based on the standard M16-platform. Barrett offers the rifle in both commercial and law enforcement/military configurations. The “bells and whistles” are the same for both versions, the only real difference is the use of selective fire, flash suppressors, bayonet lugs and telescopic stock on the law enforcement/military models.

The weapon as offered uses a mid-length gas system developed by Mark Westrom of ArmaLite, Inc. This mid length gas system changes the location of the gas port by placing it further forward than the carbine and slightly behind the rifle. This gives two major advantages: reliability and durability. By putting more distance between the bolt carrier and gas port, lower port pressures are created resulting in the bolt assembly’s velocity being slower. This increases reliability in extraction as well as extending the life of the bolt and some of the trigger components.

The barrel is manufactured by Fred Fedderson. This 1 turn in 10 inch twist barrel is chrome plated and is currently available in 16 inch length only. Barrett has plans on offering 12 and 20 inch barrels in the near future. The commercial version has a muzzle check similar to the one used by ArmaLite, Inc. The law enforcement/military versions use the standard M16-style muzzle break/compensator

The M648 gas block utilizes a folding front sight assembly. This assembly was designed by Barrett. On the law enforcement/military version, there is a threaded collet on the edge of the gas bock to install the silencer.

The Mid-Length SIR System

The A.R.M.S. Inc. SIR (Selective Integrated Rail) system developed by Richard Swan is the solution for many problems associated with military use of this type of weapon system. The SIR system is a free-floating handguard assembly that attaches to the carrying handle by a sleeve that goes on top of the rail (thus protecting it) as well as directly to the barrel nut. This give an extended top rail that runs the entire length of the upper receiver to the front sight assembly. There are four Mil-Std 1913 rails that enable optics, laser, flashlights and whatever other accessories that may be needed for the rifle with no effect on the barrel and its performance.

Free floating a military weapon has many benefits. First would be accuracy. With nothing impeding on the harmonics of the barrel, better uniform accuracy is achieved. Additionally, the barrel remains cooler as there is significantly better air circulation enabling the weapon to fire longer on full automatic fire. Another benefit, particularly with a hot barrel, pulling down on a vertical fore grip will not cause the barrel to bend or droop. The SIR system may also have a M203 grenade launcher attached to it. All the current models of the M648 feature the A.R.M.S. Inc. SIR system as well as the ARMS #40 L emergency flip-up backup sight.

The Bolt

The bolt carrier is standard M16/AR-15. The difference lies in the bolt itself. The bolt was designed by Chris Barrett and is manufactured by Lewis Machine & Tool (LMT). The bolt uses the trademark dual extractor spring “lobster tail” extractor manufactured by LMT as it is more durable and reliable. The bolt has other similarities to the LMT Enhanced bolt but Barrett made some changes in materials and specifications specifically for the M648. The bolt has a nickel plating on it.

Configurations

The M648 comes as a full weapon with fixed rifle stock as well as selective fire carbine lower receiver with the telescopic buttstock. The system is also sold as an upgrade or a conversion kit which will include the entire upper receiver assembly and magazine. The conversions can be obtained with either selective fire bolt carriers or semiautomatic only. It should be noted that when putting a conversion on a standard selective fire carbine lower receiver, the “H” buffer should be used. This is the buffer with two steel weights and one tungsten. If the standard three steel weight buffer is used the carbine may experience bolt carrier bounce resulting in light strikes on full automatic. This is not an issue with the full length rifle selective fire lower receiver. Semiautomatic guns can use either due to the bolt group will have sufficient time to lock before the next shot is fired. There has been some discussion about Barrett eventually coming out with a piston driven mechanism for this weapon system.

Shooting Impressions

The rifle received for testing was a law enforcement/military upper receiver with a standard semiautomatic only lower receiver. The author’s own Colt M4 full automatic carbine lower was used for full automatic testing. The semi-automatic only lower receiver was equipped with an LMT two-stage match grade trigger. The optics used were the M68 Aim Point Comp II, EOTech holographic sight as well as the standard backup sights. Approximately 700 rounds were fired in total in both semiautomatic for accuracy as well as fully automatic for functionality. The magazine used was a PRI made 28-round steel magazine. The ammunition was manufactured by Remington.

Fully-automatic tests were conducted in close quarter battle conditions. Targets ranged from 5 to 25 meters. The cyclic rate was about the same as the standard M4 carbine and recoil was really not that much more. There was no problem controlling the weapon. The EOTech sight made shooting easier allowing both eyes to remain open increasing speed and accuracy. By adjusting the brightness of the sight, it was useful for both close tactical work as well as short range sniping.

Conclusions

The 6.8mm Rem SPC is without a doubt an improvement over the existing 5.56x45mm for military use. But the question is, how much better? The cartridge unfortunately has a very steep uphill battle ahead of it once the military community gets a hold of it. The weapon itself is not an issue. The M16-based design is well proven. But in order for SOCOM in particular to adopt such a cartridge for use will be a gigantic undertaking. First and foremost it has to be more than better, it will have to be drastically better and offer a large advantage. Second, and perhaps the most critical, will be logistics. The cartridge will be at best used by SOCOM operators and them alone. From a logistical standpoint, adding a new cartridge into inventory, in particular one nobody else in the service uses, can cause serious problems. Once SOCOM operators are deployed they become part of the unit they are assigned and along with that goes their supply. Having non-compatible ammunition can put the operators at risk given they do not have re-supply specific to them.

The Barrett M648 and 6.8x43mm Rem SPC ammunition performed flawlessly and accurately. Barrett has not been too quick to go into production due to the fact they have waited until the development of the rifle and ammunition was completed insuring their customers get a final production weapon and not a semi-prototype. As of this writing, after more than three years of advertising and manufacturing of barrels to fire this new cartridge, the ammunition has yet to be completed and put into full production. This weapon and cartridge will undoubtedly find a following in law enforcement and commercial shooters and competitors. As far as the military use of this weapons system, only time will tell.