P1110413

The piston-based operating system has long been the gold standard for firearms design. Sure there are other methods like roller-delayed blowback and Blish lock designs, but nothing really stands up to the simplicity and raw reliability of a piston. The AR-15 rifle was originally designed to use a “gas expansion” system for operation, but these days more and more people are getting into the piston game. I’ve had a couple people ask me the difference between long and short stroke pistons, and so here’s the long and the short of it . . .

The Basics

Hiram Maxim is credited with inventing one of the first automatically repeating firearms, his Maxim Machine Gun. The story goes that Maxim was out shooting with some friends and noticed that there was a lot of wasted energy in the recoil of a firearm that could better be used operating the action and reloading the gun. That same concept — using recoil to the shooter’s advantage — is the basis for most modern firearms designs.

The trick with using recoil to do the work is figuring out a force delivery mechanism. The most efficient way to capture that excess recoil is by using excess gasses vented off from the barrel after the bullet is already up to speed, since at that point pulling off the excess energy won’t negatively impact the bullet’s performance. But getting that excess force from the front of the barrel all the way back to the action was problematic. Early attempts used everything from a spring-operated lever such as that on Browning’s early machine gun models to a reciprocating barrel and “recoil booster” that captured the energy from the muzzle, but the best solution was the simplest and cleanest: bore a small hole in the barrel, and vent the gasses that way. That’s the way almost every semi-auto action gets its energy these days, and piston powered AR-15 rifles are no different.

Once the gasses are vented, in a piston system it’s the job of a long metal shaft (the “piston”)to move the forces from the vent to the action. The gasses act directly on the front of the piston, forcing it backwards and imparting enough momentum to the bolt carrier to cycle the action. That much is common to all piston systems, but the great debate rages on about whether a long stroke or a short stroke is best. As for what that means, let me take it one at a time.

Long Stroke

A “long stroke” piston is one where the long metal piston is physically attached to the bolt carrier. Examples of long stroke piston systems include the AK-47 rifle, the M1 Garand rifle, and PWS’ line of AR-15 rifles. I’ll use the PWS as the example in this explanation.

P1040148

With the PWS rifle, the piston is welded in place and attached to the bolt carrier. When the gun fires, the hot gasses act on the front of that long shaft (you can see the discoloration from those gasses at the front) and those gasses force the piston backwards. The piston and the bolt carrier group recoil as-one, traveling the entire length of travel and then returning to the starting point ready to start the whole process again.

The long stroke piston system is the oldest of the piston driven designs, and has the benefit of delivering the most power to the action. Since the piston is simultaneously in contact with the expanding gasses at one end and the bolt carrier at the other, force from the front of the gun can be constantly applied to the bolt carrier throughout the entire reloading cycle. However, usually manufacturers will drill some vent holes in the gas tube that houses the piston to vent those gasses off before the piston stops moving so that there’s nothing impeding the forward motion of the piston when chambering a new round.

Short Stroke Piston

While the long stroke piston system works, it isn’t ideal for every situation. The added mass of the piston attached to the bolt carrier slows down the cyclic rate of the gun, and can lead to increased felt recoil. There was a demand for a system that had the same basic concepts, but could be used for much lighter firearms and quicker cyclic rates. The man who first designed such a system was a bootlegging murderer who built his firearms while in prison: David “Carbine” Williams.

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While the long stroke piston system moves as-one, the short stroke piston system has a couple different moving parts. The piston and the bolt carrier are separate moving pieces, and they operate somewhat independently of each other.

When the gun fires, the expanding gasses act on the piston and force it backwards just like in the long stroke design. Also like the long stroke design, the piston directly acts on the bolt carrier and forces it rearwards as well. That’s where the similarities stop, though. The piston only moves back about an inch or so, imparting all of the required energy in a short period of time, and then it stops. Once the piston stops moving, the bolt carrier continues moving rearwards due to the momentum it still has from that initial shove. That momentum carries the bolt carrier through the rest of the reloading cycle, and then everything marries up once more when the bolt slams home.

The beauty of the short stroke system is that it is simpler to build and maintain, as well as lighter on the shooter since the moving mass inside the gun is much lighter. The trade off is that there are more moving parts, and these systems are especially susceptible to a condition known as “carrier tilt.” Since the force is being applied directly to the top of the bolt (instead of in-line with the bolt like the gas expansion system Stoner originally designed) the bolt carrier has a tendency to tilt. The rear bottom edge and the front top edge exert more force on the surrounding materials due to that tilt, and it has been known to wear holes in the receiver extension. For that reason most good short stroke piston systems will have an exaggerated beveled rear edge to mitigate that carrier tilt somewhat, but eliminating it completely is impossible.

The short stroke system is by far the most popular for the AR-15 rifle simply because it is easy to retrofit existing designs to use the new operating mechanism. You need a special upper receiver for a long stroke piston, but the short stroke piston can be inserted into any upper. There are even retrofit kits available to let you do it yourself at home. It is currently in use by guns like the Adams Arms piston guns (their operating guts shown above), as well as the M1 Carbine, the SCAR rifle series, and others.

Which is Best?

I’ve never been able to perform a legitimate test to determine if there is a winner, but in my mind the long stroke piston is the better option. There are fewer parts to worry about, and the heavier moving mass feels more solid to me. Short stroke systems seem to be more finicky — there’s one short stroke rifle I’m testing right now where I need to send it back because it just didn’t work. That has never happened with a long stroke gun. Then again, there is more recoil to deal with, so that might be a dealbreaker for some.

It all boils down to personal preference in the end. There are companies that make great rifles using both systems, and you need to decide for yourself which one you prefer. But now that you know the difference, hopefully that decision will be more informed.

[Email your firearms-related questions to “Ask Foghorn” via thetruthaboutguns@gmail.com. Click here to browse previous posts]

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76 Responses to Ask Foghorn: Short Stroke Versus Long Stroke Pistons

    • If it was the gold standard wouldn’t blow-forward rifles be more common than piston rifles? At the very least more common than DI rifles.

      Also, we haven’t seen how well a blow-forward action would work withrifle cartridges

  1. My CMMG 6.8 SPC Spec II and Ruger SR -556 shirt stroke pistons uppers have a combined total of 1 jam in about 3,000 rounds. And that was mostly my error, since I left the Ruger piston in position 1 while I switched from M855 to Herter’s 55 grain .223. Switched to position 2, where it pretty much stays, and no subsequent issues.

    No issues with carrier tilt either, although my piston round count isn’t terribly high.

      • I also have an AA piston system,and I love it. Only 1000 rds through, no failures,too early to tell. I noticed their replacement pistons were EXPENSIVE,close to 100 bucks. My FAL’s was only 20 bucks. Is there a believable reason for the cost difference?

  2. The other question is which system has the biggest negative impact on accuracy. In the long stroke case, you have a lot more weight moving back and forth, which could shift point of aim up and down. In the short stroke case, you have less mass moving, but there is a rather explosive collision where the piston strikes the bolt carrier (assuming the bolt carrier is bumped right up against the piston when the gun fires).

    Many people swear by the DI model over either piston system citing accuracy claims. I’ve never been able to really validate if they are right or not.

    • One of the areas where a piston system might be more accurate than a DI system is in how the gas tube fits into the key.

      The AMU found over the years that how the gas tube is bent forward of the castellated nut affects how the tube hits the key, and that can affect accuracy. There are all manner of fiddlings that armorers do to make the tube fit into the key consistently and smoothly, and this level of hand-fitting of the tube into the key is removed by a piston system.

      • I thought piston guns are inherently less accurate due to the short stroke piston impact with the BCG, and the movement of more mass along the axis of the barrel creating a greater adverse effect on barrel harmonics.
        Maybe it’s just my piston guns having chrome lined barrels. Or shitty triggers.

        • A crappy trigger is always the first thing I go after. Your chrome-lined barrel would be the second place I’d recommend a change.

        • Depends on the particular barrel in question.

          When barrels are chrome-lined, they are bored/rifled oversize, then plated back to the groove/land dimension they should be.

          The trouble is, there can be variances in the thickness of the plating around and down the length of the bore.

          The best, most accurate barrels are drilled, reamed, rifled and then air-gaged to keep the diameter variation down the length of the barrel to 0.0001 (one ten-thousandth of an inch) or less. That’s a tall requirement to try to achieve in a plated bore – one ten-thousandth is a very small thickness indeed.

        • From data taken by benchrest shooters, yes. It results in easier cleaning, longer barrel life and possibly higher muzzle velocities.

          The downside is the higher cost. Chrome plating is very cheap, all things considered.

        • What about stainless steel barrels? If a quality barrel is used will there be a difference between stainless steel and “regular” steel that has been plated or melonite treated?

        • Difference how? In accuracy, both stainless and chro-moly can be quite good. In terms of cleaning ease, both can be quite easy if they were lapped to a fine finish.

          Stainless barrels are now the default among benchrest and other accuracy shooters, partly because they’re wearing a bit longer against gas erosion than cho-moly barrels.

    • To Jim Barrett,
      You wrote: “a rather explosive collision where the piston strikes the bolt carrier”
      My Bushmaster is a short stroke system, the separate piston rod is pretty close to the bolt carrier key, which is a solid key, not hollow as on a DI system. The “key” is milled not welded and is permanently part of the bolt carrier. Somehow I don’t think that is going to be a problem unless I do something absolutely stupid like put 30k+ rounds through it in a single day without letting it cool down and even then, I doubt there’s going to be a problem with the design.

  3. Personally speaking, I’d rather just use a rifle in the configuration it was intended to be used in. Piston AR’s are neat, but I’d rather stick to a rifle that was designed from the get go to use a piston.. Not one that was retrofitted on.

    • There are several ARs that were designed from the get go as piston ARs rather than gas ARs with an after-market piston system fitted on.

    • So, I take it that none of your ARs have such recent innovations as collapsible stocks, free float handguards, or flat top receivers? Because, you know, the AR-15 platform was never originally designed for such things. They happened as the platform evolved.

      The problem is not what it was or was not originally designed for. The problem is that the AR-15 has characteristics that make switching to a piston system somewhat problematic. Let’s not fall into the trap of “the old way was the best way” dogma.

      • Handguards, flat-tops, collapsible stocks sights etc. do not affect the core functions of the machine. Piston do. That said, piston or DI is a choice best left to the individual shooter and his or her needs and wants.

        • I don’t know that I agree with that. Switching buffer tubes meant swapping buffers, and that is absolutely part of the core function of the rifle.

          Similarly, I would argue that the change from “DI” to “piston” is not so dramatic as you’re making it out to be. In a DI system, the bolt carrier is de facto acting as the piston. You’re simply moving things around a bit. I mean, consider what’s involved with an AR piston conversion as compared to an AK DI conversion… the change doesn’t seem so huge in that context.

      • Changing the butt stock and hand guards are like changing the grips on your handgun and giving it night sights. Changing it from DI to piston, is like adding a safety onto a Glock.

      • “So, I take it that none of your ARs have such recent innovations as collapsible stocks, free float handguards, or flat top receivers”

        That is a false equivalence. Those things became a necessity as the fighting rifle evolved to accomodate optics and lasers, which are the new KISS when it comes to the infantryman’s standard arm.

        Pistons don’t improve the reliability of the AR platform. They’re a marketing gimmick and a good idea that has been proven no better by actual real world experience. They do, however, add parts and weight to the AR and change the motion of the recoil impulse, which are all disadvantages.

  4. I have been very satisfied with my Sig 516. I feel no more recoil that I did with my mil-issue M4/M16, and cleaning it is a snap. Hell, the cleaning is have the reason to have a piston AR!

    • Have you take out the piston yet? Mine took quite a bit of scraping. Overall, I still think pistons are easier to clean and maintain, but it seems that ARs and scraping are an unavoidable combination.

      • On my Lwrc SPR it’s not a piston, but a cup you need to clean. Soak in a little hoppes while you clean the rest of your gun and you are golden. I do the same with the piston and the gas control selector in the Scar 17s. Never needed to scrape any of them. I’m not familiar with the sig so I don’t know if that’s feasible. I use an old medicine bottle for small things and a tupperware container for bigger things. I reuse the hoppes until is gets too dirty. I’m able to put the lids on both containers while soaking. Keeps the fumes down.

      • I have found that my Adams Arms piston doesn’t need a lot of attention. It appears that carbon will build up to a certain point, then stop. My guess is that the carbon buildup raises pressures in the piston cup, which then prevents any more buildup. Kinda self cleaning.

        • What pisses me off is that PWS changed to an adjustable gas block, for “quieter operation”, never mind the fact that adjustable gas blocks are not needed for long stroke pistons and effectively undue the self regulating characteristic inherent to long stroke. 🙁

          disappointment. Talk about fixing what isn’t broken.

  5. I suppose if you want to stretch the definition of “piston” a bit, the operating rod on an M1 is a piston and it always seems to be getting bent, cattywampus . etc. So I’d want to be sure AR piston designs didn’t have similar problems.

  6. The idea that there is less recoil due to the lower weight of moving mass in the SS system seems off to me. Not saying this as someone who has fired both enough to tell, but just struck me as odd from three science of it while reading.

    Isn’t the same basic amount of mass being propelled at the same velocity whether it’s SS or LS, just the SS mass is then stopped in 2 “stages” kind of as the piston stops first and then the BCG a moment later? If I’m understanding that right, the actual recoil energy should be the same. What might change a little is felt recoil because of the “stage” effect.

    Again, not speaking from experience, more looking for clarification as a prospective purchaser. (PWS vs. BRO… what to do, what to do…)

    • 5.56 recoil is not a concept that I can wrap my mind around. One can shoot an AR with the butt pressed against one’s tender bits without endangering generations of shooters yet unborn. There are many good reasons for choosing DI over piston or vice versa, but it’s hard for me to accept that recoil is one of them. YMMV.

      • Yeah… I’m definitely not in tune with rifles to really be able to judge recoil. I know ARs are softer than my AK, which is softer than my .32 Win Special, which is softer than my 12ga… and that’s about it lol. Could be a useful mental exercise to figure out for an avid competitor though, and I find it interesting either way.

      • Here’s a table of recoil velocities, energies and perceptions:

        http://www.accuratereloading.com/recoil.html

        The “standard” for recoil used to be the .30-06 with a 150gr pill. In something like a Garand, this could come out to about 17 to 18 ft-lbs of energy, and was considered the point where recoil started becoming objectionable for some smaller shooters.

        Where’s the .223? Down at 3.8 ft-lbs of energy.

        OK, I like to talk about shotguns. Let’s talk for a moment about shotgun loads: The typical trap load is 1.125 oz of shot at about 1200 fps. That results in a recoil energy from a 7.5lb shotgun of about 23 ft-lbs – more than the .30-06 with 150gr bullets.

        How many rounds of this recoil do trap shooters shoot? Well, every round of trap is 25 rounds for singles, 50 for doubles. In trap competitions, the best shooters might shoot hundreds of rounds in a single day. The very top shooters in ATA events might end up in a shoot-off for the top spot, and I’ve seen these go on for upwards of 500 rounds – above and beyond all the shooting these competitors already did to complete the full course of fire (singles, doubles, handicap) in the competition.

        This is why I maintain that .223 recoil, especially out of a semi-auto with a buffer weight, is simply not an issue. My brain hears “recoil” and “.223” and goes “whaaaa?”

    • “The idea that there is less recoil due to the lower weight of moving mass in the SS system seems off to me.”

      because its wrong honestly.

      Short stroke systems violently strike the bolt carrier group in order to propel it backwards, which inherently gives it a sharper recoil impulse (and 223 thankfully has wimpy recoil. lets be honest for a second). Anybody see the slow mo videos of the 416? the bolt carrier group literally slams backward as far as the buffer and spring will possibly move. Not good for the long term.

      As opposed to long stroke systems that are propelled by expanding gasses rather than a mechanical strike. Softer characteristically. Although I think recoil wise they are both “inferior” to the stoner system that propels its recoil directly backwards.

  7. Good article, thanks. I’ve been looking at short-stroke piston ARs for 2 primary reasons:

    1) Much easier to clean over DI
    2) Less expensive than long-stroke piston ARs (and money is a definite issue for me)

    Recoil in a 5.56 platform isn’t a significant issue, and I suspect the difference in any impact on accuracy still falls above my own level of accuracy.

    I’m actually pretty interested in a VZ-58 too (rather than more traditional AK variants), which is also a short-stroke system.

  8. Can somebody clarify this:
    Since the force is being applied directly to the top of the bolt (instead of in-line with the bolt like the gas expansion system Stoner originally designed) the bolt carrier has a tendency to tilt.

    In a DI AR-15, the gas tube vents the gas at the key on the top of the BCG, pushing it back.
    In the one short stroke AR-15 I’ve seen (an LWRC), the piston was in the same position as the gas tube, went through the same hole in the upper, and pushed a nearly identical key. The face was slightly different to accept a piston rather than gas, but the height from the BGC and the angle were identical to the naked eye.

    I’d actually be interested in trying a piston conversion for my suppressed 300BLK pistol, because the DI system vents so much crap into the chamber that I have to clean it after 200 or so rounds. But after my experience with the LWRC, where the piston system broke twice, and was a pain to clean, I’d want to make sure I get a good one.

    • Do some reading on this. Everything I have read says that 300blk will not function properly in a piston system. I am no expert, so ymmv.

        • Hmmmm…I translate your answer to that it “will not function properly” unless you want to make it your second occupation.

      • Don’t know where you get that idea. The 300BLK works great in piston AR — tuning the gas/buffer/etc. is the same that you have to do for any custom/tweaked AR build. In fact, if anything, AR’s built for 300BLK are better in pistons ARs since they do a better job of mitigating gas blowback when you put a suppressor on them.

        The PWS 300BLK uppers run nicely. The easy to reach, adjustable gas block makes switching back and forth between super-sonic and sub-sonic ammo simple, quick, and easy.

    • If you pull apart your bolt from the carrier, you see that the bolt has what are effectively piston rings on it. The gas coming in the key gets ported down into the carrier, and the carrier is pushed backwards off the bolt on the axis of the bolt/carrier – not by gas entering the key. The carrier moving backwards causes the cam in the bolt to rotate in the carrier, rotating the bolt to unlock from the barrel extension. Then the inertia of the carrier going backwards pulls the whole assembly to the rear – by this point, the key is off the gas tube, and there’s no way the gas pressure is pushing back on the key.

      With a piston system that pushes back on the replacement of the key, and there is no force being applied to the axis of the bolt or carrier. All the force is off-center on the replacement for the gas key.

      • Interesting, I guess I’ve never looked at it that closely. I’ll take out out this weekend and see what’s in there.

        Thanks.

        • OK, for you and anyone else who hasn’t ever cracked open a BCG, here’s a diagram:

          http://www.ar15.com/content/guides/parts/boltCarrier.gif

          Part 5 is your carrier.

          Part 4 is your bolt.

          Part 3 is the cam.

          You see the key on top of the BCG, right? There’s a hole at the back end of the key, just before you get to the first staked screw (going rearward) that holds the key onto the carrier. That hole allows gas that was pushed down the gas tube, into the key, to exit into the carrier. When the BCG/bolt is closed into the barrel extension, the “piston rings” on the bolt are rearward of the two holes on the right side of the carrier (the two holes set vertically on the right side of the carrier). The gas pressure blows the carrier to the rear, which causes the cam pin (3) to force the bolt to rotate, which allows the bolt to come out of the barrel extension. The travel of the carrier vs. the bolt is limited, also by the cam pin, so that after the bolt rotates to the unlocked position, the rearward inertia of the carrier will pull the bolt out of the barrel extension.

          So: When you’re cleaning an AR, you need to pay special attention to the bolt recess in the carrier, the “piston rings” on the bolt, the rear portion of the bolt itself as well as the gas key. They all need to have powder residue removed from them.

          The way you tear down an AR BCG is to pull the cotter pin (1), drop out the firing pin (2), then pull the cam pin (3) out of the carrier (5), which will allow you to pull the bolt (4) out of the carrier (5). If you want to tear down the bolt to remove the extractor and ejector, that will take a pin punch to accomplish.

  9. Direct Impingment = $h!Ts Where It Eats, and dumps all sorts of heat back into the receiver.

    The men at COP Keating will tell you that everyone of their DI M4’s failed over the course of the sustained firefight there in 2009, largely due to carbon build up and excessive heat. Now granted, most of us won’t be engaged in a deliberate fire fight against a determined opponent… or will we?

    My Adams Arms (Short Stroke Piston) upper fires right at 1 MOA all the way out to 500 yards with 69 grain rounds loaded over 24.5gr of IMR4895.

    • Guess what other weapons shit where they eat, suppressed M14s and Scars, MK46s and 48s, 240s but you don’t hear people complaining about that.

      As for Camp Keating, they had no failures with they weapons, it was the Battle of Wanat, not Kamdesh that the greatly exaggerated reports of M4 problems come from. As for Wanat the independent review found soldiers using they weapons in ways no carbine could keep up with. One of the AARs from that found soldiers were firing they rifles at rates that would melt the barrels of belt fed weapons. One soldier reportedly dumped his first 9 mags in under a minute and half, that’s over double the sustained rate a 240 is capable of. He wasn’t even sure what mag his gun stopped working on.

      Unfortunately their is no magical gun that would have changed the outcome of Wanat. Poor leadership from the higher ranks in allowing the situation to be so bad at the site, and burnt out and poorly trained soldiers resulted in they disaster, not their rifles.

  10. Another bonus of the AA short stroke system is for those running suppressed. When the gas exits the AA piston sleeve, it has to do a 180 degree turn, thus slowing the gas down. In long stroke systems, you still get the supersonic crack from the exiting gas.

    The AA design also benefits from the gas sleeve cleaning itself during operation.

    The long stroke tube gets hotter and dirtier.

    The only downside to some Short stroke systems is bolt carrier wear on the extension tube. That is, unless they have it angled properly to minimize this. Newer SS systems have corrected this.

    • I disagree on both counts. Those are advantages that long stroke has over SS.

      -long stroke systems generally dont need regulating dials to adjust the gas settings. SS does.
      -long stroke systems are inherently more “self cleaning” (i hate that term, but bear with me) because the gas piston rubs in the gas tube. Since the LS has more mass, that means any ice, dirt or other debris is blown out or pushed somewhere else.
      -long stroke systems are more mechanically simple. Less small parts and less breakage.
      -they’re equal in terms of “cleanliness” or lack thereof. Gas and carbon have to go somewhere.

      Edge; long strokes. By far (although every AR piston conversion company loves short stroke for some odd reason. I think its equivalent to the “bridge jumping” justification)

  11. I thought that the usage of the terms long and short stroke were correctly meant to describe the duration of time that the gas acts on the piston and that using it to describe the piston being attached to the BCG or not was a lay bastardization of the terms.

    ex. the AK is actually a short stroke design as the gases only act on the piston for the length of the gas block.

  12. All self loading actions will pass gas over the extracting brass and dump residue into the action. A blowback .22, a delay roller lock HK, a Colt M16, or an AK of any variety will get filthy bolts and trigger groups with a dozen mags at the range.

    What most don’t realize is that the modern intermediate cartridge lets the shooter pay less for ammo and shoot more of it. The average range session of a Battle Rifle caliber gun was less than 100 rounds, often just fifty, much the same as the basic load most carried in combat. The M16 – 300. You shoot more ammo, you are going to see more residue.

    Don’t confuse that with how the action works – because the M16 has very little extra gas dumping into the action. What shooters do is compare the simple bolt cleaning of a piston gun with the active piston in the bolt carrier of the M16. Lay out all the parts side by side after shooting equal amounts of ammo and then speculate which is dirtier. Compare a piston with the ring section of the bolt of the M16, compare the interior of the bolt carrier with the cylinder of a piston gun, etc.

    Of course, it’s going to take MORE work to tear down a piston, and the Garand is a classic example – if you fail, you can’t. The gun becomes incapable of disassembly without major repair, if forced, and a gunsmith is required to get the piston out of the cylinder. In the military, forcing it apart was a punishable offense because it was a known issue and it would damage parts. There were mandatory limits on how much ammo you could shoot before maintenance was required.

    Pistons have improved since then, but in no way are they less dirty or have any inherent ability to resist the buildup of gas residue. What has happened is that a large number of experienced shooters from the WWII era have pased on and the remaining generations are much less experienced or skilled in the use of different firearms. They depend on the internet for information because there aren’t one in ten who served and who could be around in family gatherings to ask. Now it’s one in one hundred – and so myth and out right falsehoods keep getting propagated by those who equate their masculinity with how well they argue their misinformed view on the internet.

    All self loading actions pass gas around the brass as they extract it, and they all get residue in them. Blatantly ignoring that evidence to preserve their social standing or save face is not spreading the truth, it’s spreading misinformation and it doesn’t do anyone in the shooting community any good. It’s also quite laughable to those of us who have shot quite a few of the worlds weapons and then had to clean them. That alone teaches the real world experience more than someone who claims otherwise on the net.

    • Exactly right.

      The dirtiest guns, IMO, are the blowback .22’s when they’re using wax-lubed ammo. A congealed mess of wax, unburnt and burnt powder ends up coating just about everything within an inch, maybe an inch-n-half of the breech.

      Another thing I’d add to your piece above is that on the Garand/M-14/M-1A is that the oprod can (and does) get bent, requiring finesse and care to straighten it, or it will need to be replaced entirely. This will likely be true of the piston systems for the AR that have long “oprods” or “push rods” in the mechanism. Cleaning the piston on the Garand is easy enough today, but with WWII-era lubes and cleaning fluids issued to the field? Well, not quite so easy. Today, I just pull the plug, spray in some MPro-7 and let it set. A little compressed air, some carb cleaner, more air and we’re pretty much off to the races without having to really tear it down. The same formulation works with the AR BCG and bolt; it is just a whole lot easier to pull apart the BCG & bolt on an AR than it is on a Garand/M-1A.

      The central issue in both types of rifles is the choice of powder for the ammo. Choose the wrong powder, and you’ll be doing a lot of cleaning – in both types of actions. Choose the right rifle powder for your loads and you’re rewarded with much less cleaning workload. People who refuse to learn to reload don’t get much choice. Yet another reason to reload…

  13. Hey Nick! quit calling my distant relative David Carbine Williams a murderer! It was just a negligent discharge! Enjoyed the article.

    • I don’t want be rude but if he was in prison for murder I would guess he is a murderer.

      BTW that reminds me of a joke:
      A man is in court for having killed a man. He states that he was simply careless. He says he was peeling a banana (with a knife) and threw the peel away and that the victim stepped on it, slipped and landed on the knife. When the judge questioned that story by stating that the victim had 20 stab wounds the accused simply said “I ate 20 bananas that day”.

  14. correction:
    It was John Browning who noticed that the muzzle blast was blowing the grass around. Then he harnessed that energy to work the action, and the Browning “potato digger” machine gun was born.

  15. Also, while it is true that the short stroke is lighter and simpler, the DI system is lighter and simpler still. The biggest problem with the ARs DI system is not the DI action, but the fact that the ARs system vents the gases directly into the locking lugs, which are small and sensitive to the grit that is being blown directly into them.
    A much better DI system is seen in the Swedish Jungmann, which uses the rear of the carrier to lock, leaving no moving parts at the front to fail when dirty.

  16. I see no reason for owning a gas piston AR.

    You are spending more money on a rifle that is no more reliable in the short or long run, but which has proprietary parts and adds more moving parts on a system that already works very well.

    But, buy whatever you want I guess. Its your money. Im just saying that you will not be missing out on anything if you spend money on any quality AR (BCM, noveske, DD, LMT, etc)

    Whats better? the old design.

  17. Somehow I managed to sit here and read this entire article and all of the replies. I have read about the difference of these types of systems and I noticed there is one thing that I didn’t see anyone bring up. I guess I don’t know if it is true or not, but isn’t there some loss in pressure behind the round due to any of these systems forcing the BCG back to reload the next round and eject the casing? I mean, the entire purpose of this discussion is based around semi-automatic reloading concepts. I guess I just feel like more pressure has to sacrificed to force the BCG back on a DI system than on either of the piston styles. This is pressure sacrificed from effective impact force spread out across a distance.

    For example, while hunting you expect to have a certain hit power to ensure bullet penetration on your target is strong enough to effectively penetrate the outer body to reach the vital organs. If you are pushing the edge of the effective range of your rifle/ammo to make a kill on a certain type of game, then would the piston style system allow for a greater force at that distance, or is the sacrificed power so nominal that it makes little to no difference? Also, with some of these piston style systems, they not only allow for quick changes from sub-sonic to super-sonic types, some also allow for the ability to turn the gas piston of completely. This means that no pressure is lost behind the round to reloading, but it disables the semi-automatic action of the rifle. If there is no pressure loss/gain, then the only purpose of this would be for saving brass, and to me that seems pretty unimportant in the long run. Unless you are making illegal kills and trying to hide evidence…

  18. Long Stroke vs Short Stroke vs Direct Inpingement… this is an old one,

    Note that no action impacts accuracy at all, either positively or negatively. This is because the bullet is already out the barrel and on its merry way before anything begins to reciprocate.

    Long Stroke Pistons use a single piston, flush with operating rod, tappet, and what have you, attached to the bolt, which is actuated by gas pressure and reset into position by a single return spring.
    Pros:
    * Simplest design
    * Most reliable overall
    * Longer operation makes recoil more of a predictable push than a sharp jump.
    Cons:
    * Adds a small amount of weight to a weapon.
    * Longer recoil pulse.
    * Parts breakage requires replacement of whole unit.

    Short Stroke Pistons have a small piston that is actuated by the gas and then transfers momentum to the operating rod, which goes to the bolt.

    Pros:
    * Allows faster cyclic rates than LSP.
    * Parts can be more easily replaced piecemeal.
    * Shorter recoil pulse.
    Cons:
    * Jumpy recoil due to all the parts moving very quickly.
    * More prone to wear and breakage than LSP.
    * Heavier than DI.

    Direct Impingement uses just the gas hitting the bolt, which may or may not have a small internal piston of its own.

    Pros:
    * Lightest weight.
    * Mechanically simplest system.
    * Smooth recoil pulses.
    Cons:
    * Extremely prone to unreliable operation.
    * Gas system hard to clean.
    * Lack of reciprocating parts means that recoil is not partially absorbed by the action.

    From best to worst: Long Stroke, Short Stroke, Direct Impingement.

  19. Correct me if I’m wrong, but carrier tilt only affects short stroke gas systems when used in AR15s. Other rifle systems (Vz 58, Bren 805, SCAR, etc) use bolts of completely different design and mass (generally heavier and bigger), and I’ve never heard that they suffer from carrier tilt. I believe the problem is caused by trying to fit a operating system in a rifle that wasn’t intended for it. And that bolt wasn’t intended for it.

    What would happen if someone tried to take an Kalashnikov and cut the bolt down and turn the whole thing into a gas impingement system — I’m sure it would be very unreliable.

  20. Is it correct to say that carrier tilt is unique to ar platform as other platforms don’t have a moving carrier that impacts a buffer assy? Since a long stroke has the piston attached to the bcg (I would like to know whether it means long piston rod or long duration also) does long piston travel inside a tube or carrier that could possibly mitigate carrier tilt? Also doesn’t gas sustem and length of barrel have a lot to do with (barely) felt recoil? The 16″ carbine I shot had way more recoil than the 14.5″ mid length and I would assume an 18″ or 20″ rifle length would shoot smooth also.

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