Direct Impingement vs Gas Piston: Key Differences

Direct Impingement (DI) vs. Gas Piston, whether short or long-stroke, has been the topic of many campsite fires and cookout conversations.

When you compare these two systems, you naturally will have people with very different opinions. The community is split since everyone has their own beliefs about which operating system is superior. Of course, if they say they favor the direct impingement system, they probably own an AR-15.

To help you decide which side of that debate you fall on, this article will compare each system and shed some light on how they came about.

What Is Direct Impingement?

Direct impingement uses the pressurized gas from a fired cartridge to force the action back directly.

When a round is fired, the gasses used to propel the round forward are fed through a gas tube and into the gas key of the bolt carrier group. In the expansion chamber, it pushes the bolt carrier group back, ejecting the spent round and chambering a new one.

And then the party starts over once again.

Most would believe that the direct impingement system came out in the 1950s when the Armalite platform was introduced, but the first DI firearm was introduced in 1940, which was the French Rossignol ENT B1 rifle.

Direct impingement would become popularized in 1964 when the M16/AR-15 platform went into service with the U.S. Military.  However, the early M16s were known for their unreliability and subsequently got plenty of people killed.

That would prompt an overhaul of the platform as a whole, which included upgrades to the direct impingement operating system.

It took time, but today, the DI system is favored for being a lightweight and reliable platform used in American conflict for over 50 years.

What Is a Gas Piston?

Whether short or long-stroke, gas piston systems work similarly to direct impingement but use the gasses from the spent round to drive a piston into the bolt carrier group, ejecting the round and chambering a new one.

It’s a simple design that results in firearms that will function under extreme situations. You’ve probably heard of some firearms that utilize this design, like the AK-47, AKM, RPK, etc.

The long-stroke gas system used in the AK-47, M1 Garand, and Bren light-machine gun (to name a few) uses the mass of the long piston attached to the bolt carrier group to rip that round from the chamber.

These firearms tend to be over-gassed to ensure reliability. They also have more recoil and are often heavier.

Short-stroke or free-floating piston systems are similar to the long-stroke system, but the piston is not attached to the bolt carrier group. Instead, the piston uses kinetic energy to bang against the bolt carrier group and eject the round.

Short stroke systems are used in the M1 Carbine and the SKS and are sometimes seen on Armalite platform rifles.

Function

Of course, just as we mentioned above, one of the main differences between the direct impingement and gas piston-driven operating systems is how they operate.

Direct impingement uses the gas from the fired round and forces some of it into the gas tube, pushing it into a gas key and into the expansion chamber, which expands and sends the bolt carrier group back into the buffer.

Gas piston-driven firearms use a free-floating (short-stroke) or fixed (long-stroke) piston to drive the bolt carrier group.

The difference between the two is that the short stroke system moves separately from the bolt carrier group. There are different ways that this may occur.

For example, the M1 Carbine has a piston directly contacting the bolt carrier group. On the other hand, some firearms, like the SKS, use a connecting rod along with the piston so that operation can occur.

Either way, both use kinetic energy to operate.

Long-stroke gas pistons, like those you see on the AK-47, are attached to the bolt carrier group. When the round is fired, the rod and bolt go back simultaneously, ejecting the round and back to its original position.

These genius designs have been around longer than most of you reading this (and definitely longer than the guy typing it, me.) This is one of those “if it ain’t broke, don’t fix it” deals.

They may have made advancements in the designs and have tweaked them, but Stoner and Kalishnakov’s designs have remained as the foundations.

Cleanliness

Piston-driven systems are cleaner but heavier than direct impingement systems. Parts usually last longer on these systems due to the materials used and cleanliness.

With gas piston systems, less gas enters the system when the round is fired. We should thank the Lord for this because AK-style rifles are gassier than that one time you thought coffee and tacos were a good combination.

The gas that isn’t used on piston guns is released in the gas block, which keeps the internals of the rifle cleaner than direct impingement.

I’m not saying they don’t get dirty at all; I’m just saying you won’t have to clean them nearly half as often. Are you picking up what I’m putting down?

Direct impingement systems are lightweight but dirty, requiring more cleaning, and parts wear over time. All that gas gets dumped back into the system. That’s why your hands are so dirty when you break down your AR-15.

Long story short, gas piston-driven firearms are way cleaner. And more reliable.

Longevity

Some AK-47s that have been around since the first crate left the warehouse are still in use today. These piston-driven guns have a history of outlasting almost any other firearm worldwide.

That has to do with the materials used and how little the gas piston system deals with the stresses of running dirty, which plagues shooters more than rats did in Europe in the 1300s. Carbon buildup can really get in the way!

So if longevity is what you seek, get a gas-piston firearm. Or, take care of your rifle.

There are many operating systems in the firearms industry, but few have achieved what direct impingement and gas piston systems have. While these designs have been tinkered with and tweaked to perform as they do today, their function remains mostly the same.

Personally, I have more experience with direct impingement firearms, thanks to the Marine Corps. So, DI will always have my heart. As will Luciana, my dirty girl M16A4, who made me fall in love with the Armalite platform.

Which one do you prefer in your rifle, and why? Let me know in the comments.

Take care now.