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The AR-15 rifle is a gas-powered rifle. This means that a small portion of the gasses following the bullet down the bore are tapped off. The power of these gasses is used to operate the firearm, and gas operation comes in two styles: Direct Impingement (DI) or Gas Piston. Both of these systems have their pros and cons, and this blog will explain the two and examine their differences.


How Does an Operating System Work?

Any self-loading firearm has an operating system. The operating system is the functional, mechanical system that is responsible for performing the eight steps of firing. Known collectively as the “cycle of operations,” these eight steps are feeding, chambering, locking, firing, unlocking, extracting, ejecting, and cocking. If any of these functions fails to occur, a stoppage occurs, and the gun is no longer functioning as designed.

“Direct impingement” and “gas piston” are two operating systems that perform this cycle of operations. Each has its own set of benefits, and though they work slightly differently, both essentially perform the same task: delivering power to the bolt to unlock, extract, eject, cock, letting the buffer spring push the bolt carrier forward and feed a fresh cartridge, beginning the cycle of operations again.

What is Direct Impingement

The Direct Gas Impingement System (DGIS, also called Direct Impingement or DI system) is the original operation system of the AR-15 platform, as designed by Eugene Stoner, the inventor of the rifle. Direct impingement means that gas is carried directly back to, and enters, the bolt carrier group. As the bullet is traveling down the barrel, it is followed by rapidly expanding propellant gasses. Once the bullet passes the gas block, this gas encounters a small hole in the top of the bore. This small hole allows a small amount of gas to pass upward into the gas block.

This gas is routed into the gas block, where it is directed into the gas tube. The gas tube carries it into the gas key of the bolt carrier group. Once in the bolt carrier group it acts by applying rearward pressure. The bolt is forced to rotate thanks to the bolt cam pin unlocking itself. Once the bolt unlocks, it travels rearward, the extractor pulling the spent brass out with it. Once the brass clears the chamber the ejector flips it out of the ejection port. The bolt continues traveling rearward until the hammer is cocked.


Benefits & Disadvantages of Direct Impingement

Direct impingement works because gas acts – or impinges – directly on the bolt without intermediary or stand-off. As with any other system there are some benefits and disadvantages to direct impingement gas systems. Let’s take a look at them.


Benefits of Direct Impingement

One of the main benefits of DGIS is that it just works. One of the main complaints about a direct impingement rifle is that it may become dirty. While it’s true that this does occur, the direct impingement system is outstandingly reliable. It has stood the test of time, serving the U.S. military and the militaries of dozens of other nations well for half a century.

The direct gas AR-15 rifle works best when thoroughly lubricated. The M6 and AR-15 platform and its spin-offs like the M-4 carbine and Mk12 Special Purpose Rifle are all DI-operated rifles. All have performed with astounding reliability in all theaters including during the global War on Terror. The direct impingement system just works and works and works.

The direct impingement AR-15 rifle is also lighter than its gas piston counterpart. The gas block and gas tube add negligible weight to the AR-15 upper receiver while, as we will see, the additional components of the gas piston system incur a big cost in weight.


Disadvantages of Direct Impingement

One notable disadvantage of the direct impingement system is when firing is suppressed. Suppressors slow the escaping gasses to reduce muzzle report. This holds the gasses in the bore longer, which allows more gas than necessary to travel backward into the bolt carrier group. This can cause the gun to be “gassy,” that is, expel a lot of gas around the shooter’s face. It can also deposit more gas into the bolt carrier group but again, properly lubricated the AR-15 will work. If you don’t shoot suppressed these factors are irrelevant to you.


What is Gas Piston

A gas piston AR works a little bit differently. The rifle still has the same underlying cycle of operations, but the way the gas is managed to power the bolt is different. When the rifle fires, the same propellant gasses launch the bullet down the bore. Once again, some of these gasses are tapped off and forced up through the gas block. Instead of being routed through a gas tube and directly into the bolt carrier, they are intercepted at this point.

The gasses instead act upon a “gas piston.” The gas piston is an operating rod that applies the rearward pressure to the bolt. The gasses push this piston to the rear and are then vented out the front of the gun through a gas port. This keeps dirty gasses away from the moving, internal components of the rifle.


Benefits & Disadvantages of Gas Pistons

Advantages of Gas Piston

A gas piston gun may have an advantage when firing suppressed, or when frequently switching from suppressed to unsuppressed fire. Because gas doesn’t directly enter the bolt, the additional gas that is tapped when firing suppressed isn’t an issue. It is vented out the front. Gas piston guns also sometimes come with an adjustable gas block with settings for suppressed or unsuppressed fire. This can allow you to lower the amount of gas operating the system when firing suppressed, which reduces wear-and-tear on the gun.

Gas piston guns will also shoot cleaner and cooler. This is not the huge benefit as it is made out to be. These guns still get dirty and require maintenance, and direct gas ARs still work just as reliably as piston guns.

Disadvantages of Gas Piston

One major disadvantage of a piston-driven AR is weight. A piston gun will be heavier than a DI gun, all other things being equal. There is much more weight in the upper receiver of an AR-15 rifle with a piston. Rather than a lightweight, stainless steel gas tube, the upper receiver must accommodate a piston system. The piston system will include a gas block, and the piston itself, both of which will be bulkier than the parts necessary to keep a DI gun running.

Gas piston AR-15 rifles are also more difficult to service. Piston driven ARs are still in the minority. While any gunsmith can get an AR up and running, the institutional knowledge (to say nothing of parts) is not as widely available for gas piston guns. Parts are also peculiar to the make of gun and may or may not be interchangeable with other manufacturers’ parts. Again, this is a big contrast to the direct impingement system, whose parts are nearly universal. And for the serious user, having replacement parts on hand may be difficult.

Another huge downside of piston driven ARs is accuracy. Gas piston guns just aren’t as accurate. During fire, the heavy operating rod is moving back and forth. This causes flex of the barrel, impacting its harmonics. This is not to imply that direct gas guns are completely inaccurate – some have proven to be really accurate – but they will never be as accurate as a direct gas gun. The movement of the operating rod back and forth also creates a greater recoil impulse in a piston-driven AR.


U.S. Arms Company AR-15s

If you want the finest AR-15 money can buy, buy a U.S. Arms Company rifle. All U.S. Arms AR rifles are direct impingement.. For ultimate reliability and accuracy, our AR rifles are available with mid-length, rifle length, or carbine-lengths direct impingement systems. The mid-length gas system is ideal for barrel lengths of 14 to 16 inches, allowing enough gas to function the gun reliability, while not over gassing it , creating undue wear and tear,  and creating unnecessary additional recoil.  A rifle length gas system is for a barrel length of 18” or longer. The carbine length gas system is shorter, allowing more gas into the operating system, for when reliability in adverse conditions is the overriding concern.

All of our rifles are made with premium quality components and are best-in-class. Rifles are offered in 5.56mm for a flatter shooting rifle, or .300 Blackout for better terminal ballistics in the close-quarters environment. Our upper and lower receivers are machined from 7075-T6 aluminum, and each set is individually mated. All of our bolt carrier groups are FailZero nickel-boron and are rated for full-auto fire for the ultimate in dependability.

With U.S. Arms Rifles – our UTAW, UTAW Pro, or UTAW Gen II Champion, in 5.56×45 or .300BLK – you can rest assured you will get a reliable, accurate, beautiful rifle, right out of the box. And you can get it in your choice of color including basic black, or Cerakote solid colors or a variety of camo. U.S. Arms Company makes the best AR-15 rifles that money can buy.