A Comparison Between Internal and External Piston Systems

Around the 2004 time period with the introduction of the HKM4 (HK416) the industry as well as the military have been on a kick that somehow the external short or long stroke piston systems were more reliable than the M16/M4 direct gas systems. Well, is it???

I can recall the time when this debate began, the regular Army was very happy with the performance of the M16/M4 series rifles. They were functioning well in both Afghanistan and Iraq. In fact the M4 was so well received that the Army stopped procurement of the M16A2 and A4 in favor of a pure fleet M4 carbine for all ground elements. Reports came back (Soldiers Perspective on Small Arms) that the M4 was the most preferred of all the small arms, and that relatively few had any major issues, not that there weren't criticisms. The top critique was that soldiers wanted a larger caliber cartridge which they felt would increase stopping power. In my opinion, the real problem was the poor performance of the M855 ball cartridge. Other rounds have been proven to be extremely effective such as the Mk262 Mod1 and Mk318 SOST rounds. The Mk262 Mod 1 ammunition is used by SOCOM in MK12, MK18 and M4 carbines. Due to the poor performance of the M855, the Marine Corps have developed and issued the MK318 ammunition. This is a barrier blind projectile which improves the ballistics and performance majorly over the M855.

The vast majority of complaints came from the SOF community. Looking back into that time, around 2003, the SOF units had complaints from barrels blowing up and bolts breaking. Well, when the facts came to surface it was found the SOF guys were firing large volumes of fire, which the M4 was not designed for. The M4 was designed as a lightweight carbine to replace the M1 carbine and M3 SMG. This is not a LSW! This was not a design flaw in the rifle but misuse. The bolt breakage had a little blame to go around. Colt had some quality control issues over a year but the main cause of bolts breaking was excessive operation in SOF units. While doing research, many SOF did not have their weapons routinely maintained. Bolts were not replaced in reasonable intervals as well as barrels.

Operators would take the same rifles they trained with into combat. They had no idea how many rounds were through the bolt or barrels. To deal with the unique needs of SOF, the M4A1 was developed. It utilized a Crane designed heavy barrel to assist with the high volume fire breaking engagement scenarios. Colt had fixed the bolt issues. The rule of thumb was you replace the bolt when you replace the barrel. These corrections produced a M4A1 that was durable and reliable enough that it killed the Navy MK16 program.

The long/short stroke operating system is not new, it has been around since the turn of the last century. Really WWII showed the first military application of semi-auto as well as fully automatic high power rifles. The long stroke piston is that of the AK-type family of weapons. The operating rod, piston are both attached to the bolt carrier. As the cartridge is fired, the projectile passes a gas port in the barrel. up a port in the front sight base/gas block and into an expansion chamber created between the front of the piston and the rear of the gas block. The high pressure drives the bolt carrier/operating rod assembly rearward causing extraction and ejection. The short stoke patent is similar accept the operating rod/piston are a separate component. In this system (SKS, FN FAL, G36, StG44) the operating rod/piston are pushed rearward, the face of the operating rod strikes the carrier driving it rearward while the operating rod returns to its fully forward position by that of a spring. Both systems confine a majority of the carbon deposits to remain in the gas block area, not in the bolt mechanism.

Of the external long/short stroke piston you have two basic types of piston. One solid piston and the second which utilizes gas rings.

The top shows a standard piston, this one happens to have a conical shape, most are flat. Below shows the gas rings. The standard AK-type rifle uses the top. In order to make this type cycle properly you must over gas the system. The piston is not air tight sealed so gas escapes around the piston so to compensate you introduce more gas in the system. By using gas rings, you seal off the chamber which requires far less gas to operate the mechanism. Also by using less it is more friendly for suppressor use.

Utilizing the external piston in either of the configurations shown produce a reliable operating mechanism. These systems are heavy and the operating rod and gas block used will interfere with the harmonics of the barrel which can effect accuracy. Due to the components, this system is more prone to freezing in extreme cold conditions particularly when ice is present. Overall system is heavy. As previously stated the fouling remains in the front of the piston.

The direct impingement or what I like to refer to as internal piston offers several benefits. This system differs in the fact there is no operating rod. There is an exhaust pipe, if you will, that directs gas into a carrier key located on the bolt carrier. The key directs the gas into the piston/expansion chamber which is created between the back of the bolt and the back of the bolt carrier. This gas creates a hammer like blow driving the bolt carrier directly rearward inline with the shooters shoulder.

As the bolt unlocks, unneeded gas is vented from the system via vent holes in the carrier. The direct gas system holds the barrel, bolt carrier group and recoil mechanism directly inline with the shooters shoulder decreasing muzzle climb and recoil as opposed to the external piston rifles which recoil off center of the shooters shoulder. This system is much lighter and less sensitive to ammunition pressure variations and due to no moving parts in the front of the rifle, this system is more reliable in the ice conditions. Also due to the gas tube, theoretically the rifle will be more accurate, especially when using a free floating barrel.

The down side is that the expansion chamber is inside of the bolt carrier. Deposits of carbon and fouling are introduced into the bolt mechanism. This requires more cleaning than the external piston systems. Let's be straight with this, these systems will certainly go 1k rounds without cleaning so I do not want to overstate its maintenance.  Like any professional, you clean your rifle after every use. Soldiers maintain their weapons after every engagement or firing session regardless of what weapon they have. I prefer to use the term reasonable maintenance. All three of these operating systems are all methods to complete the same task, knock the bolt to the rear for extraction and ejection.

I want to now address the weapon systems. The external piston system was never meant to be put on an AR-platform. The inline construction and direct push of the gas rearward to unlock the bolt and push the carrier directly into the buffer tube. When you introduce a operating rod to an AR-platform you normally increase gas flow and then have carrier tilt due to the operating rod striking the bolt carrier off center. The bottom edge of the carrier damages the receiver extension/buffer tube. When you look at a rifle which uses external piston such as an AK or AR18 you will see the bolt carrier rides on rails to keep it aligned properly. The AR receiver does not do this, not necessary in the direct gas system.

Some companies have put ski's on the bottom of the rear of the carrier, as well as moved the bottom of the receiver extension forward; so that when the receiver closes, the rear of the bolt carrier is already in the receiver extension and does not have to "jump". Another symptom of this use of external piston system in the AR is the gouge behind the cam slot in the upper receiver. Again, due to the different operating mechanism of the external piston vs direct gas unlocking stroke.

There are certainly very good external piston versions of the AR (LWRCI, POF, Sig). All do experience the mentioned issues. Myself, I appreciate both systems, and I own all of them. People do ask me my opinion on this topic quite often. So here it is. I prefer the direct gas system in an AR-platform. A properly made rifle is every bit as reliable as an external piston. I clean all my guns after every shooting session. For an external pistol, I prefer rifles that are designed around it, such as: the FN SCAR, SIG MCX, CZ 805 BREN, FAL and so forth.

It is my opinion that many of the complaints with DI rifles malfunctioning are caused by out of spec rifles. Professionals maintain their weapons. I have AR-based rifles that shoot sub MOA. And you can't ask for more. Some of the external piston rifles in M4 platforms rifles have experienced durability issues due to the higher cyclic rate they require to maintain reliability in case of the HK416. Several others have issues with gas system components breaking. These higher rate of fire guns wear even faster with use of suppressors. If using an external piston AR, if possible, have a gas valve for going from suppressed to unsuppressed.

This is a very opinionated industry. Everyone has their individual flavor. People have different needs as well. When you look at military procurement of small arms throughout the world you see just how different a nations needs are. Special Operation Forces is even more interesting. Some nations prefer homegrown weapon systems, while others want compatible weapons with other SOF. Look at the UK where their military uses the SA80 rifles and the SAS use Diemaco C7 and C8 M16-type rifles. Even the Chinese SOF use a Chinese copy of the M4. Recently Vietnam replaced its aging AK's with licensed production IWI ACE rifles chambered in 7.62x39mm.

For the average end user, you pick your poison. For someone who is using these weapons in the real world you must be a little more selective in your choice. Myself, my go to rifles would either be LMT or earlier Colt rifles. I prefer these because they are combat grade rifles. If I was to want an external piston, I would look more towards the Sig MCX, FN SCAR, or even a Robinson Armament XCR. Many modern MSR rifles are made well, just test the rifle out before you rely on it.



11/06/2017 5:48pm

Good intro to the topic. Reference could have been made to the M855A1 developed by the US Army, or to the 2015 Aberdeen magazine tests which compared performance of the M27/416 to the M4A1 - but, again, good intro. If I have a new shooter curious about external vs internal pistons for the AR, will send them this article.

Brent Sauer
11/07/2017 8:26am

Great article that is very informative. Thanks for your work!

Benny B
11/09/2017 12:41pm

Great article Chris. Your observation about how the AR is designed around internal piston configuration and therefore isn't the ideal platform for external piston implementation sure makes sense.


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    Technical Specialist, 10 years in forensics, veteran, technical writer and armorer instructor.