Tam on pistons …

Tam says (referring to the RAA PDS) …

Look, if you’ve just gotta have a gun with a piston in it so your friends will know you’re cool, get a gun designed from the ground up to use a piston, not some kludge affixed to a Stoner-type bolt carrier assembly that was never intended to have a foot-long lever torquing it from the top.

Read the full post and the comments here.

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • Jesse

    My sentiments exactly.

  • HCE

    I just sold my 2 year old LWRC M6A2 for three reasons:

    1) Gas block failure. I mean FAILURE. 2 of 4 bands were cracked all the way on the left side of the block. On the right the rear band was cracked 90% of the way and the forward right band was maybe 20% cracked. LWRC paid for shipping and replaced this GB with one from the PSD model. (Why didn’t they do that in the first place? I didn’t spend $2000+ on a rifle to be a beta tester)

    2) Op rod spring cup kaboom. The op rod spring cup was shattered by the “head” of the op rod. This appears to be due to the fact that the op rod spring was peened down and unwound a little at the head end, allowing the op rod’s head and op rod spring cup to meet while the action cycled.

    3) The LWRC was merciless on brass, tearing rims off or nearly tearing rims off on even 10% from max pressure loads.

    4) All of this trouble for a $600 premium and 2 extra pounds and a single source for critical parts like Op Rod related bits.

    I don’t think so.

    So I sold this rifle and built up a nice DI gun around LMT components at just over 1/2 the cost of the LWRC. I could not be happier.

  • Victor

    I agree, if you’re gonna use a piston syste, buy yourself one that was designed for it, not some conversion kit.

  • M14 type rifles have pistons, they also fire a useful caliber, and are cheap… well they’re cheap in Canada any ways; I guess they’re not as cheap in The States.

  • Vitor

    That was some bitter ranting.

  • Lance

    IE buy a Ruger or H&K don’t wast money on a hybrid.

  • drewogatory

    The only benefit to a piston system, IMO, is that in a life or death situation you can crank the gas valve WFO so that it will cycle no matter what (of course, you’ll probably just rip the rim off of the jammed cartridge and screw yourself even more but…). Given that I and most civilians will never ever ever have this problem, why not stick with cheapest,simplest solution? The only failures I’ve had with the platform have been from incorrectly crimping the .50 Beowulf, hardly the rifles fault. Luckily you can just grab a frigging fallen branch off the ground to clear it, that bore is like the Holland Tunnel.

  • Vitor

    Piston or DI, the PDS at least offers a foldable stock in an AR package, quite convenient for those who want to carry a guy on some bag.

  • jdun1911

    Piston AR has their own set of problems. However a lot of AR piston owners doesn’t know until they either abuse or have a high round counts.


    Do you have pictures of the damaged parts. I’m interested in them.

    People keep saying that piston runs cooler than DI. Yes it runs cooler on the bolt and BCG however the heat is still there just at a different place. It doesn’t magically disappear.

    The heat that is normally deposited in the BCG is now in the piston and Op rod. So is the carbon. With such little clearance is no surprise to see those parts fail much sooner.

  • Stella

    The response to the proliferation of piston ARs by some is reminiscent of the insecure having their sexual identity challenged: I have no idea why anyone is so threatened by variations in a product.

    I have owned a piston AR made by LWRC for a few years. Its fun to shoot and much easier to clean than its DI counterparts. With smart shopping I even got it for an equivalent price. I just don’t see the problem.

  • charles222

    Besides that cleaning a DI AR is just as easy and that it costs half as much?


    Getting a DI AR to an acceptable level of clean is maybe fifteen minutes of work; clean out the star chamber, swab the barrel, pop the bolt and clean out the inside of the bolt carrier. Done.

    ARs should not be made squeaky-clean; they run (as all firearms do) better with a mild amount of carbon on the moving parts. Not caked on, but just barely there to a finger touch. I’ve managed to keep my M4 to about that standard for a few years now and I’ve never suffered a jam.

    HCE: OUCH, man. That sounds like a horrible time, particularly on a 2 grand plus rifle.

  • Andy

    DI for my .44 AMP AR. In 2011 I’ll have a .458 SOCOM and a 6.5 Grendel. Both will be DI.

    Do I think that a “from the ground up” piston AR is better? I do. But not enough to think I’m risking my life with DI.

  • W

    interesting how i never see photos or any concrete evidence of such “catastrophic events” occuring with gas piston AR15’s. My LWRC M6A2 has had 10,000 rounds plus through it and still functions flawlessly.

    Based on the general lack of evidence and general DI fanboyism, i remain extremely skeptical of the “flaws” of the gas piston system. Interesting that USSOCOM continues to use the gas piston HK 416 and the US military is adopting the M4A1, despite a few ranters arguing against gas piston designs.

    considering gas piston has been around since john browning, i am rather convinced it functions when needed and offers damning evidence to why a overwhelming majority of rifles use a gas piston design.

    Oh a side note, the problem i encountered in the military is improper PMCS’s and PMI. Soldiers tend to over lubricate M16s/M4’s, which results in CLP collecting more dirt and fouling the weapon. proper cleaning and lubrication of the bolt assembly and bolt (to ensure ease of rotation) and star chamber results in a reliable M4 carbine. The utilization of Tetra gun lubricant, militech, or any other superior product (with light lubrication) will ensure the M4 operations adequately. Also, the utilization of superior magazines significantly improves the weapon’s reliability.

  • charles222

    The only portion of SOCOM that uses the HK416 is Delta, due to a requirement for short-barreled suppressed weapons, which is what piston ARs were invented for in the first place. The M4A1 does not have a piston at all, so I don’t know why you’re even bringing that up.

    And, yeah, AWG, John Noveske, heck, the entire British military with their recent adoption of the LMT DI AR over piston-equipped competitors from H&K and Sabre Defense=”fanboy ranting.” Cause the Asymetric Warfare Group is just a bunch of civilians posting on the internet. John Noveske, why, he must not know anything about firearms at all; I guess his mom…or secretary…or uh, some random homeless guy…must be responsible for his company’s success.

  • W

    yes, Delta has 416’s and they have been repeatedly used by the US Air Force branch (pararescue) and the US Army’s asymmetric warfare group (who had theirs taken away). The Ranger regiments utilize the SCAR, which is also a gas piston design. The US Marine Corps M27 IAR is being field tested. Numerous countries’ militaries and special forces are currently using the HK 416.

    About the “M4A1”, the US Army is upgrading the M4 rifle from existing M4A1s with a heavier barrel, heavier rails, and a gas piston operation. Perhaps the rifle will have a designation of M4A2? or something. Though it does not matter. http://www.armytimes.com/news/2010/08/army-seeks-better-carbine-082810w/

    “And, yeah, AWG, John Noveske,”

    who i hold in the highest regard, though i will maintain my contention that they are traditionalists like many other AR15 users, who favor the fewer parts for DI rifles. There is no evidence that they wont delve into the gas piston market either…

    “heck, the entire British military with their recent adoption of the LMT DI AR over piston-equipped competitors from H&K and Sabre Defense=”fanboy ranting.”

    Yes, way to attempt to derail my highlighting of fan boy rantings by drawing to a already established point. Yes the British military adopted the LMT, which is the L129A1 7.62mm rifle, a fine weapon. Why didn’t they choose the H&K 417? i have no clue, and you don’t either. Consider the number of countries adopting the H&K 417. Consider also the British military also adopted the Enfield SA80, which is a atrocious rifle in my opinion. I would be more concerned with the credibility of the British defense industry (which is why the soldiers in the british military that actually engage in firefights utilize the M4 or US weapons ((unless you count the conventional forces that are stuck with the SA80)).

    “Cause the Asymetric Warfare Group is just a bunch of civilians posting on the internet.”

    yes, which would explain their reluctance to give up 416’s. Interesting. Interesting how the military units that are the most engaged with hostile forces favor gas piston designs…but im sure, emulating your humor, that Delta Force is a bunch of amateurs.

    “John Noveske, why, he must not know anything about firearms at all; I guess his mom…or secretary…or uh, some random homeless guy…must be responsible for his company’s success.”

    hmmmmm, if you want to make a point, you dont have to be sarcastic. Just because noveske doesnt have gas piston ARs right now doesnt mean that he wont. Also comparing a top of the line AR15 that utilizes the finest machined parts available to one mass produced on a industrial scale (colt or bushmaster) is shallow to say the least.

    Dont like the fact that gas piston designs are favored by a majority of the world’s military? too bad. they are. Not everybody things DI is the best thing ever invented.

    (ill make it a point to everybody that i own a custom AR15, which is DI and has a noveske barrel as well as a gas piston LWRC M6A2…i like both weapons though i understand the advantages of a gas piston system for battlefield conditions. Perhaps the most credible opinion comes from those who have fought in the dirt of Iraq and Afghanistan utilizing direct impingement weapons).

  • Tam

    I find it interesting that my point is somehow misconstrued as being an expression of traditionalist love for the blued steel and walnut of the direct-impingement AR…

  • Chris

    Many have seen this, but I figure it was related. The author is quite qualified in his, “in the field,” background.


    There is also a nice pros and cons list here from a reputable company.


  • jdun1911

    Armytimes.com are not affiliated with the US military. Armytimes are HK fanboys to a point that people thinks they are on HK payroll. I take anything that is reported by Armytimes with a grain of salt.

    • jdun1911, I don’t think that is fair. They report what the commanders are saying and they appear to be quite pro-HK.

  • W

    interesting how many of the world’s elite paramilitary and special operations forces appear to be “pro HK”…including military commanders, who represent the higher echelon of education and intellect in this country.

    i have no grudge against direct impingement designs. I think with proper education and training, they can have excellent reliability. Not all DI guns are created equal; for example, comparing a noveske rifle to a DPMS or bushmaster is not exactly comparing apples to apples.

    The main disadvantage of gas piston designs is not mechanical but logistical sort of speak. with dozens of different designs, there is a lack of commonality, though i have yet to see the effects of the so-called “carrier tilt” in my gas piston AR15s. Mechanically and in practice, gas pistons are clean, reliable, and effective weapons because heat remains on the piston instead of the parts where the bullets cycle (bolt and bolt carrier). Due to the wide range of AR15’s of varying quality, comparing DI to gas piston is near impossible.