The Failed Promise of the “Piston” AR-15 Conversion

A few years ago, piston/op-rod conversions for AR-15s were all the rage.

We were bombarded with propaganda about how unreliable the standard AR-15 was. We were led to believe that our rifles, which had worked just fine for years, were suddenly obsolete with the introduction of a spigot, op rod, and other parts to replace the gas tube, that we’d have the “reliability of an AK-47” as a result. Many conversions became available in a matter of months.

And for a while, everything seemed fine. Until, that is, people started putting rounds downrange with them.

Most of the conversions were poorly designed, and quite a few people had broken parts as a result. You see, much more force was being put on certain parts – such as the gas block and the bolt carrier – due to additional reciprocating mass. This also has the effect of increasing felt recoil and in some cases led to barrel whip, decreasing accuracy.

Most people are unaware that every AR-15 is piston-operated. The tail of the AR-15 bolt acts as a piston. What the AR-15 truly lacks is an operating rod. You’ll see me refer to “op-rod AR-15s” in this article – I am referring to what others may call piston ARs.

In order to understand why such a conversion isn’t the best option for a standard AR-15, you need to know how an AR-15 works. I’ll try to distill it to one paragraph.

Gases from the fired case (which expands, by the way) travel back through the gas tube, for a specific amount of time based on the distance between the gas port and the muzzle – we refer to this as dwell time. Does this gas simply “push” the bolt back? Not exactly. Gas travels inside the gas key of the bolt carrier and expands rearward, forcing the carrier back, while it also pushes the bolt forward. As the carrier pulls back, the cam pin moves in its slot in the carrier, causing the bolt to rotate and unlock. At the same time, the gas that’s still in the barrel is keeping the case expanded to fill the chamber. Once that gas is vented out the front of the barrel, the case shrinks, allowing the entire assembly to pull back while the extractor continues to grip the rim of the case.

Now, the piston conversion. Some of these items vary based on the exact conversion, but this is a general overview.

As soon as the bullet passes the gas port, gas enters the gas block and pushes against whatever components the individual manufacturer has decided to place in the way. The effect is that the operating rod pushes against the top of the bolt carrier – and instead of having the gas enter the carrier and exert pressure parallel to the bore fore and aft, the rod hits the carrier key – or modified one piece carrier – which has the effect of causing the bolt carrier to move at a tail-down angle. There isn’t any gas pushing the bolt forward while the carrier unlocks, so the carrier just pulls the bolt back and forces it to unlock from the locking lugs of the barrel. The piston/op-rod assembly, which is under spring pressure, starts to move forward to its “at rest” position, however, in some cases it will slam forward with enough force to shear any pins holding the spigot in place – that’s why you won’t see many standard FSBs on piston conversions any more.

I mentioned the carrier moving back at an angle – why is this bad? Well, it rubs against the buffer tube at the 6 o’clock position. No big deal, right? Well, my Ares conversion had enough carrier tilt, as it is called, to knock the unstaked castle nut loose, which allowed the buffer tube to rotate out of position when I twisted the rifle while the stock was still in my shoulder, which caused the buffer retaining detent and spring to fly out from their normal position – which stopped the weapon from functioning. This was obviously an extreme case, and a very compelling reason for a staked castle nut, but it was disconcerting, to say the least.

So, do op-rod AR-15s have a place at the table? Yes, they do. You’re probably confused, because it seems like I’ve been bashing them for a long time now. Well, these rifles do work very well in short-barrel configuration, when dwell time isn’t long enough for the AR-15 to work properly. I’ve owned 7.5″ and 10.5″ AR uppers that functioned perfectly, but if I were to do it over again, I’d buy an LWRC if I was going to 10.5″ and under. I believe that 11.5″ SBRs, with properly sized gas ports and proper weight buffers, will offer the same reliability when compared in numbers and over the long term. I do not feel that this is the case with 10.5″ and under barrels. Many folks who shoot SBRs also use suppressors, and those folks tend to migrate towards LWRC and similar brands. It’s not always the case, but a reduction in blowback is a pleasant change for high volume suppressor shooters.

That’s not to say that a suppressed SBR sans op-rod won’t function. Many shooters have reported 0 malfunctions in high round count carbine courses with such setups, and often suppressed LWRC SBRs get just as filthy as their standard counterparts.

Still don’t believe what I’m saying? Well, I’ll defer to those with far more experience than I. I’m unaware of any experienced instructor who oversees carbine courses on a regular basis who is an advocate of piston/op-rod conversions on barrels longer than 14.5″, and most will say 11″ and under. Even those like Larry Vickers, who was heavily involved in the development of the excellent HK416, feels that they are unnecessary for unsuppressed standard carbine applications, and continues to teach classes using, in most cases, a Colt or Daniel Defense rifle without an op-rod.

This article went way longer than I had planned. However, if you walk away with a better understanding of how an AR-15 operates, and a greater appreciation for its exceptional reliability potential, as well as the usefulness of op-rod ARs in niche applications, I’ve met my goals. Thank you for your time.



Filed under Firearms

11 responses to “The Failed Promise of the “Piston” AR-15 Conversion

  1. Mike W

    Awesome article, great reference point. Thanks.

  2. Brandon

    Good points, but I still don’t like the idea of a rifle the shoots gas (and dirt) directly into the action.
    When someone develops a proper op-rod gun (like the HK416) is it at a disadvantage compared to direct gas? assuming costs are equal

  3. Excellent article. I agree 100%. In addition, the tiny locking lugs on the AR bolt were not designed for direct mechanical impingement. The lugs are unlocked while the bolt is pinned in the forward position by gas pressure. If you look at an AR bolt, there is very little wear on the back of the lugs. Re: Brandon’s question/comment above. There’s nothing wrong with an op-rod gun, but it should be designed from the ground up like the FN Scar, the Robinson Arms, or the Magpul (or whoever makes it now) Masada. Retrofitting Eugene Stoner’s design is just a plain bad idea.

  4. Neil

    Thanks for the article.

    However, there are a lot of errors regarding the technical aspects and actual use of the systems.

    Price comparisons are flawed as well.

    Pat’s T&E was done with 2007/2008-era LWRCs. You fail to note that even in the younger stages of life, they still performed well and since then have been modified and upgraded with better engineered parts, coatings, and designs.


    • 87gn

      Feel free to highlight any errors, I’m not looking to spread misinformation, especially about a quality manufacturer. Prices, as you know, have been fluctuating. I’m still unable to find a NIB M6A2/M6A3 for under 2 grand, save a brief sale by one vendor.

      Is there substantial data that shows “revised” LWRC weapons will outperform their 07/08 counterparts from a longevity standpoint? I do believe that I implied 20,000 trouble free rounds as being nothing to sneeze at…

  5. Kir

    FWIW, the LWRC does not use a ‘heavy’ barrel. It uses a gov’t profile barrel without the M203 notch.

    • 87gn

      LWRC has apparently changed their barrels a number of times. Regardless, it’s still significantly heavier than comparable non-piston/op-rod carbines in the price range.

  6. rachid

    I was wondering what you think of the PWS piston system? I understand the piston (with piston) vs gas key/carrier as the piston, but the PWS system has a piston that is attached to the key like the AK. So I’d assume the carrier would not tilt if the piston is still inside the tube for a long stroke.

  7. gunblobber

    Nice post. Thanks for explaining everything so well. I never realized the full path that the gas takes inside the AR-15 bolt carrier — I thought that it was more or less just a piston system without a piston. Nor was I aware of all the issues with the op-rod AR’s now being offered. As somebody who just bought a Sig 556, I am relieved to find out that I made the right choice. The 556 operating system is pretty much pure AK (look at my latest post for a comparison of the bolts and carriers; I will be doing a more detailed post about the takedown of the 556 soon) despite the external similarity of the lower to the AR-15.

    Anyway, I just read your entire blog over the past couple of days. Nice work! Your posts have been very informative.

  8. jon

    great article but i disagree with you saying direct gas impingment is comparable to a gas pistons reliability,i might not be a gun journalist but i can say from personal experience the piston operates more brother has two gas impingment ARs one colt and the other bushmaster,both costing over $1,300 each.we did a test with his well maintained ARs and my new LWRC m6a2,and it involved stacking our clips with different ammo not just one bad included wolf,and surplus LWRC m6a2 chewed through all five clips easily with no malfunctions,the same can’t be said for my brothers guns with four malfunctions from the bushmaster and three for the colt out of five 30 round clips stacked with different brother was a little upset after that test because he takes really good care of his rifles.we didn’t do an endurance test because we like our guns to much.i can also shoot .80 minute all day long with optics at 100yards as well can my brother.

    • 87gn

      I’ve fired as many as 3,000 rounds through a DI AR in a week with no malfunctions. Something is wrong with your brother’s ARs.

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