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It may be a coincidence that this technical note came across our e-transom after Ruger announced sales of piston-driven uppers for AR platforms. Or it may not. In either case, informed debate makes all sides stronger. Vive l’evolution!


PURPOSE: To compare the merits of internal and external piston drive systems.

FACTS: The AR-15/M16 and the AR-10 family of rifles employ a unique gas powered operating system patented by Eugene Stoner in the 1950s. This gas operating system works by passing high pressure propellant gasses tapped from the barrel down a tube and into the carrier group within the upper receiver, and is commonly but incorrectly referred to as a “direct impingement” system.

The gas expands within a donut shaped gas cylinder within the carrier. Because the bolt is prevented from moving forward by the barrel, the carrier is driven to the rear by the expanding gasses and thus converts the energy of the gas to movement of the rifle’s parts. The bolt bears a piston head and the cavity in the bolt carrier is the piston sleeve. It is more correct to call it an “internal piston” system.

Most previous semiautomatic rifles use an “external piston” system operating in a gas cylinder mounted outside the receiver, but instead attached to the barrel. Propellant gasses expand within the cylinder and force the piston to the rear. The piston either contacts a rod that and drives a carrier to the rear (FAL), or are part of, connected to, or strike a rod segment that passed around the action to cam and move the bolt (M1, M14, AK- 47, SCAR). In some cases the piston is fixed and the movable cylinder drives the rod (AR-180).

ADVANTAGES AND DISADVANTAGES: The Stoner system provides a very symmetric design that allows straight line movement of the operating components. This allows recoil forces to drive straight to the rear. Instead of connecting or other mechanical parts driving the system, high pressure gas performs this function, reducing the weight of moving parts and the rifle as a whole.

In external piston systems, the path of the operating force is mechanically shifted around the action, resulting in a considerable mass of moving parts moving outside the centerline of the firearm and producing various torques within the system.

There is a common belief that the external piston operated systems are less accurate than the Stoner internal piston system because the operating parts start moving while the bullet is still in the bore. This is not true: Army Ordnance tests conducted in the 1960s revealed that the bullet is 25 feet out of the bore of the M1 and 15 feet out of the bore of the M14 before any operating part begins to move. It is more likely that the imbalances of the external piston, operating rod, cylinder, and other parts hanging on the barrel produce disruptive vibrations as the bullet exits the bore.

Although movement of the operating parts while the bullet is in the bore isn’t apparently a culprit in reducing the accuracy of external piston systems, the inherent accuracy of the Stoner internal piston system has been consistently confirmed in competitive shooting. In all events that allow use of any mechanism the shooter wishes, the Stoner internal piston system is prevailing. Few competitive shooters use the Garand or Kalashnikov systems, and none observed now use the FAL system. In American Service Rifle and NRA competition, the external-piston operated rifles are considered a significant disadvantage

There is a debate about which system remains cleanest. The internal piston system tends to leave propellant residue in the receivers, while the external piston systems keep the residue outside the action in the cylinder. External piston driven systems, however, tend to allow more external dirt into the action because of the openings required for various connecting members (operating rod, bolt lugs, etc). Comparison tests of the M16 and the M-14 before Desert Storm confirmed the superiority of the Stoner system in sand and dust tests, and recent testing has proven that proper cleaning of either system provides excellent reliability.

ArmaLite concludes that there is no technical advantage to an external piston system employing current ammunition. It will likely, however, provide external-piston systems to the market as customers demand.

As a last note, misuse of the term “direct impingement” to describe the Stoner system is so common that it has confused the issue. A direct impingement system like that of the AG-42 Ljungman or the French MAS-49 rifle taps gas at the barrel and passes it into the receiver in a way similar to the M16, (the source of the confusion) but deposits it into a small, shallow cup or pocket in the carrier. The gas expands there and drives the carrier to the rear with relatively little pneumatic advantage. The addition of the Stoner internal piston system provides significantly more pneumatic advantage to the rifle and little of the blast of escaping gas at the breech end of the gas tube of the earlier rifles.

Mark Westrom

NOTE: Santa Farago has sent scribe Chris Dumm an ArmaLite AR-15 and FN SCAR for test and evaluation. Watch this space . . .]

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  1. hahaha – “It will likely, however, provide external-piston systems to the market as customers demand” … humm, sounds like someone is behind on development, doesn’t it? FYI Mr. Westrom, it is called “direct impingement” because the exhaust gases directly impinge on the carrier bolt. If there were no advantages, particulalry in sandy environment, and is less accurate, then why was the SCAR brought into the military? “External piston driven systems, however, tend to allow more external dirt into the action because of the openings required for various connecting members (operating rod, bolt lugs, etc)” – Apparently Mr. Westrom has never even seen a piston-driven system. He should check out the LWRC youtube video where they pull it out of a bucket of water and shoot a full 30-rd mag to empty. They do the same test with Iraqi sand, mud, etc. Comparing how clean a M14 stays vs a M16 is not exactly apples-to-apples. I wonder why he didn’t compare how clean a new SCAR or LWRC rifle stays compared to an AR-15. Don’t get me wrong, I love the AR platform – but the advantages of a pisto- excuse me – “external piston system” are many.

  2. I’m glad that the proper use of the term ‘direct impingement’ has been clarified. Armalite does not dwell, and with good reason, on the AR’s sensitivity to the very combustion residue which Stoner’s design deposits directly inside the crucial ‘internal piston’. The AK may allow more debris inside the receiver cavity, but there is so much empty space in there that functioning is rarely affected. The AR bolt and carrier are machined to the tolerances of a Swiss watch, and are far too susceptible to fouling and lubrication-related malfunctions.

  3. Comparison tests of the M16 and the M-14 before Desert Storm confirmed the superiority of the Stoner system in sand and dust tests,

    Were these tests of reliability or just to see if there was dirt in there?

  4. Many M-16 ‘dirt and sand’ tests are conducted with the dust cover closed, as in “Close Dust Cover. Expose Weapon To Sand And Dust. Test-fire Weapon. Record Results. Repeat.” This is great, but M-16 dust covers don’t tend to stay closed in combat, and no other modern infantry rifle needs a manually-operated dust cover at all.

    Are we really that surprised when a manufacturer says “Our tests prove our product is superior?”

  5. External AR piston systems suffer from carrier tilt as the piston impacts the carrier above it’s plane of travel. Carrier have been enlarged in the rear to reduce the tilt, however the tilt pressure still exists and results in increased friction between the carrier and buffer tube. I have replaced my piston drives with SCARs as they employ an inline piston drive.

    The aforementioned letter sounds like armalite will be introducing a piston drive soon. Lol

  6. Rab- I don’t know of a single manufacturer that still have a problem with carrier tilt, do you know of any? Ruger, with the SR556, and a hand-full of others had issues, with many rifles being hit or miss. Today, even Ruger has improved op-rod designs and the SR-556 works fairly well (although I still won’t buy one!). For most manufacturers, it was a matter of simply moving the contact point on the carrier (where the op-rod “pushes”) rearward.

  7. Patrick,

    Every manufacturer suffers from carrier tilt. All they can do is mitigate the tilt itself by widening the boss of the carrier, but they can not eliminate the pressure appossed to the plane of travel. Not even LWRC can change the laws of physics.

    I used to be a big proponent of AR piston systems until I understood the system better. I have sold off my LWs in favor of the SCAR which has an inline piston system.

    It all gets down to degrees of need/want. The average AR shooter will never push their guns close to the limits. Me, I want the most durable personal defense weapons possible regardless of how hard theywill or will not get pushed.

    • Over 4000 rounds and I have zero carrier tilt issues, or even a sign of wear for that matter.. This was one of the reasons I went with the LWRC. I agree that you cannot change the laws of physics, but you can certainly eliminate them to the point of being moot. Gas key placement, modified carriers, and quality coatings can push the maintenance or replacement interval so far into the future that most (>95%) shooters will never notice. I will admit though, I still want a SCAR-H (7.62mm). There is no doubt that the FN-built SCAR is an awesome gun, even though modifying and accessorizing is about 4000% more difficult than an AR-based rifle.

  8. I agree that 95% of shooter will not notice–I even acknowledged that concept in my reply above. Most will not shoot 4000 in a lifetime or even 1000. I have, on several occasions, shot 4000 in 3 days.

    • I hope .223 and not 7.62 – even a SCAR in .223REM @ 4000 rounds over 3 days is a bit much! Was that a FrontSight or similar course?

      • I tend to suffer from Dunning-Kroger as far as my abilities go, and I didn’t think I shot that much, but when I thought about my ammo expenditures I realized that I turned a fair amount of money into noise.

  9. TTACer says:

    “Comparison tests of the M16 and the M-14 before Desert Storm confirmed the superiority of the Stoner system in sand and dust tests,

    Were these tests of reliability or just to see if there was dirt in there?”

    Many tests regarding reliability have been run, with two in the past couple years so I won’t address them (The M16 and other rifles proved reliable when properly cleaned and lubed.)

    The tests I’m referring to were done by Army Marksmanship Unit staffers at Ft. Bragg TX. These accuracy-focused men believed that the desert conditions would prove an advantage in the M14. They frankly were a bit prejudiced.

    They found that the M16A2 had a significant reliability advantage and were surprised to find that it performed as accurately out to 800 yards as the M14 in the hands of good shooters. They came away with an unexpectedly favorable impression of the M16A2.

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