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Bushmaster Gas Piston Rifle

Sometimes operators and officers don't take care of their weapons like they should. That's why so many companies have been working on developing a cleaner M16/M4 design. And it's why Bushmaster recently developed the M4 variant that it calls the "Gas Piston Rifle."

Nick Jacobellis Headshot

When I worked as a uniformed police officer, I went on patrol armed with nothing more powerful than a Smith & Wesson .38 caliber service revolver and a backup gun. It wasn't until I joined the United States Customs Service that I was qualified to carry select-fire and semi-automatic assault rifles, carbines, and submachine guns.

The first time I qualified with an "assault rifle" I was handed a refurbished World War II M1 Carbine with a 15-round magazine. After I graduated from the Federal Law Enforcement Training Center (FLETC) in Brunswick, Ga., I qualified with a Ruger Mini-14. It wasn't until I went to Marine Enforcement Officers School at FLETC, to learn how to handle different size boats for an undercover assignment, that I became acquainted with the select-fire and semi-automatic M16 rifle. After completing this training, I carried the carbine version of the M16 when I flew drug interdiction missions and conducted smuggling investigations on the ground in different types of terrain.

I favored the carbine version with the retractable stock. I liked this variant so much I requested that my crew and I be armed with four Colt CAR-15s and one M16 with optics when we participated in a covert air and marine operation off the coast of Colombia in 1990. This was a dangerous operation that involved meeting up with a Colombian vessel 12 miles off shore to pick up two major violators and more than 700 kilos of cocaine. The Colt CAR-15 rifles that we had on board were kept at the ready at all times in the event that we ran into serious trouble in the drug-infested Caribbean or along the Colombian coast.

After I retired I was forced to sell my personally owned pre-ban Colt CAR-15 because retired law enforcement officers were foolishly not grandfathered into the Assault Weapons Ban. Fortunately, I was able to continue to own a Colt AR-15 that was not restricted under the ban.

Eugene Stoner's Dirty Design

Sorry for the long intro, but I wanted to let you know that I am a huge fan of the heavy barrel M16 rifle and the M4 Carbine with a collapsible stock. Naturally, this means that I accept the fact that the original M16/M4 design operates with a gas system that deposits plenty of hot gases and burned powder directly into the upper receiver and bolt assembly. As a result, I expect any M16/M4 that I use to require a detailed cleaning and a proper amount of lubrication to continue to work flawlessly.

If the standard M16/M4 is not kept clean and properly lubricated, it will eventually start to malfunction. Certain parts can also be damaged and break if the buildup of burned powder and other contaminants prevents the rifle from functioning as designed. With proper maintenance, the standard M16/AR-15/M4 can be trusted to provide excellent service as a military or law enforcement firearm.

But let's get real here. Sometimes operators and officers don't take care of their weapons like they should. That's why so many companies have been working on developing a cleaner M16/M4 design. And it's why Bushmaster recently developed the M4 variant that it calls the "Gas Piston Rifle."

A Cleaner Design

I'm not very technically inclined (how I learned how to fly a plane is still a mystery), but I will do my best to explain how the Bushmaster Gas Piston Rifle works. As someone who has cleaned and maintained a number of issued and personally owned M16 and M4 rifles and carbines, I have become familiar enough with the Eugene Stoner design to describe the M16/AR-15/M4 firing mechanism as being a magnet for hot gases and burned gunpowder that can cause malfunctions and damage the bolt assembly.

The weak point in the original M16/
AR-15 design involves the use of gas from expended ammunition to power the bolt to the rear as a means of cycling the action to unload and load ammunition from the magazine. Simply put, the more you shoot a conventional M16/AR15/M4, the more burned powder and hot gas is directly injected into the upper receiver. Failing to properly remove this debris can foul or retard the action and cause malfunctions during the unloading or extracting of ammunition or the feeding and firing of

In contrast, a gas piston M16/M4 operates by using hot gases and burned powder from fired ammunition to power a steel operating rod that engages the striker face key on the bolt carrier. The gas-powered movement of the operating rod forces the bolt to travel rearward to facilitate the extraction of the empty ammunition casing from the chamber while cocking the weapon in the process. Once the bolt is pushed all the way back to the rear end of the upper receiver, the compressed buffer spring propels the bolt carrier forward. This action removes a single round of ammunition from the magazine and loads it into the chamber with the bolt locked into position behind the fresh round of ammunition.

In the Bushmaster Gas Piston Rifle (and in other gas piston M16/M4s), the internal area of the upper receiver, including the bolt, bolt face, extractor, and firing pin, are kept amazingly clean and cool because a good percentage of the hot gas and debris that would normally be injected into the upper receiver is kept inside the gas piston chamber to power the operating rod. The gases and burned powder that are not used to power the rod are vented from the rifle.

Accuracy Testing

I can hear you now, you're saying, "That's great. But how does the rifle shoot?"

The simple answer is that it shoots about as well as any other M4. And it shoots more reliably.

Recently, I was invited to conduct two back-to-back tests and evaluations on Bushmaster M4 Carbines that were being used to train U.S. Intelligence Officers in the Arizona desert. During this Test and Evaluation more than 35,000 rounds were fired through two Bushmasters and four other M4s. With the exception of a few easy-to-clear malfunctions, five of the six M4 test rifles, including both Bushmasters, performed amazingly well.

We also used the occurrence of a nasty sand storm to really put the rifles to the test. As the sand storm blew through the area, I kept one Bushmaster carbine and an M4 from a different manufacturer outside for more than 15 minutes. Both M4s were laid against a sand berm, placed on the ground, and held at chest level. Their dust covers were left open and their barrels unplugged. I also decided not to clean the two test rifles, which were covered inside and out with sand, so I could determine if a Bushmaster and another M4 variant would reliably operate after being exposed to the swirling grit.

The results were startling; the Bushmaster fired 109 rounds in rapid succession with two very easy-to-clear malfunctions after being completely compromised by sand and grit. The other M4 that went through the same sand storm fired one round and malfunctioned so badly that I had to use a cleaning rod to clear the jam.

Oh, and the Gas Piston Rifle is a real tack driver, too. The sample Bushmaster Gas Piston Rifle was fired at various combat distances to determine its accuracy when compared to a standard M4. Since I have limited space to write this product review, I will limit my remarks about accuracy and simply say that the Bushmaster Gas Piston Rifle that I tested is as accurate as any direct impingement M16/M4 variant I ever fired. During one range session I was able to consistently hit a metal plate the size of the scoring area of a POST Qualification Target at different distances out to approximately 40 yards.

While not everyone who participated in the test agreed on my next point, I also believe the Gas Piston Rifle exhibited a slightly softer recoil than a standard M4. However, everyone did agree that the bolt assembly in the Gas Piston Rifle was significantly cleaner and cooler than the bolt assembly of a standard Bushmaster M16/M4 that was fired alongside of the gas piston Bushmaster. The test rifle was flawlessly reliable during three different shooting sessions, including when the Bushmaster gas piston upper receiver was attached to a full-auto lower assembly.

As far as cleaning is concerned, the gas piston system proved to be easy to disassemble and reassemble. It is important to note that the upper receiver on a Gas Piston Rifle is disassembled and cleaned like the upper receiver on a direct injection or standard M4.

Pros and Cons

One of the biggest cons of the Bushmaster Gas Piston Rifle is cost. It lists for about 500 bucks more than a conventional Bushmaster M4.

Is it worth the extra money? That all depends on how often you shoot your AR.

If you belong to a tactical team or a special operations unit that trains with M4s on a regular basis, I strongly suggest that you buy a gas piston rifle or carbine for your individual or agency use. On the other hand, if you occasionally spend some time on the range so that you can qualify with your patrol rifle or carbine once a year, I suggest you save your money and continue to use a standard direct impingement model M16/M4. Even at the current inflated prices for .223 rounds, you can buy a few cases of ammunition with the money you will save by not buying a gas piston rifle.

The Bushmaster Gas Piston Rifle is worth the expense if you need a rifle or carbine that will run cleaner and cooler after you fire a substantial amount of ammunition. The main difference between the two systems boils down to a question of endurance. By this I mean the nature of the gas piston system enables an operator to remain engaged in combat operations with less concern about fouling the action due to excessive use of his or her firearm.

And of course you may make up the extra cost of the Gas Piston Rifle in cheaper maintenance costs. Yes, the Gas Piston Rifle still needs to be cleaned and lubed, but its design should result in less expense for parts replacement. Because the bolt assembly and upper receiver are not exposed to enormous levels of intense heat and a hot mixture of lubricants and burned powder, the Bushmaster Gas Piston Rifle is not subjected to the same destructive forces that can cause excessive wear and parts breakage in its conventional counterparts.

The Bushmaster Gas Piston Rifle is not ideal for every cop. But if you work on a special unit that needs a clean, accurate, and reliable M4 for hard duty, I would recommend that you contact Bushmaster and get some trigger time with this weapon. You may decide that it's ideal for your team.

Nick Jacobellis is a former police officer who was physically disabled in the line of duty while working undercover as a U.S. Customs agent. He is a frequent contributor to POLICE Magazine.

About the Author
Nick Jacobellis Headshot
Special Agent (Ret.)
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