Well the truth is a "DI" is really an internal piston but external piston is just as good.

A lot of people talk about parts and longevities and truthfully it's irrelevant to the conversation. I believe piston type ARs came out in the 80s and never really took off until the military took interest in it at the same time the AWB expired. So people were buying crap left and right and anything that had the word "mil spec" or "pre ban" to it.

A lot of what you said is correct.  Some is not.  Specifically "lighter, less recoil, and less moving (proprietary) parts".

Lighter is not true.  SOME pistons may be heavier, but SOME pistons are not.  If your "piston" world only revolves around certain manufacturers, they are heavier.  If you look outside that small circle, there are several that are not heavier.  Also, once you add all the crap people put on their rifles, it's irrelevant.  

Less recoil is not true.  Recoil is about tuning.  Again, if you're limited to a few manufacturer's you may not know this.  You can tune a piston gun in the same way you would tune a DI gun.  If you do it right, you wouldn't even know the different.  The only way a piston gun has more recoil is if you limit the discussion to long stroke pistons that have higher reciprocating mass.  Most piston ARs (there are exceptions) are short stroke pistons where the piston does not reciprocate with the BCG.

The less moving parts is a bit of a straw man, sort of.  Yes, the op rod is a moving part.  However, that is the LEAST likely thing to have any issues in a piston gun.  An op rod is certainly less complicated to deal with than the interactions of the gas tube, gas key, bolt tail piece, and gas rings of a DI gun.  Keep in mind, most (not all) piston guns don't have a gas key or gas rings.  They don't need to ensure that seal is good to run properly.  And some (Superlative Arms) don't even require the piston spring to run.

You are correct about proprietary.  I'll give you that.  You can't just pick up an op rod or gas block from anywhere.

1. Yes, I know DI is an internal piston. We all know it's an internal piston. This doesn't need to be mentioned in every single thread about DI vs. Piston.
2. Even now, piston ARe aren't that popular, but I've seen a good bit of them here and there. DI is the most popular option but from what I've read in this thread, piston is better at certain niche applications like suppressor hosts and for finicky receivers that don't like excess grime.

6 lb 3 oz, with optic. And that's even a bull barrel.

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Nice!  I don't know what my most recent build weighs, but I know I dropped about a half pound off the weight by using a Faxon gunner profile barrel instead of a government profile.  There's no way the difference between the piston and the DI systems would add up to a half pound.  Heck, my optic weighs more than the barrel.  LOL.

Not bad, man. Really digging the lower, what is it?

I'd rather have M-LOK over Keymod, but hey, it's your gun, brother.

After reading the Henderson Defense thread, and watching the results from the iraqveteran8888 (maybe misspelled his handle there) meltdown videos of several different full auto piston guns, I am not at all convinced they are an upgrade to DI when applied to an AR-15.

Just my 0.02, worth what you paid for it.

Carrier tilt is real!

It's a Christensen CA-15 Recon.

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There was a glowing review in Guns magazine about 6 years back or so and I fell in love with the looks (Keymod was the more popular choice back then). The extremely high MSRP ($3499) kept me from getting it. But a few years later I found one brand new on clearance for ~50% off so I bought it.

People call it a botique rifle...fine. I don't claim it to be KAC or LMT tier...but it's a fine shooter, has been literally 100% reliable without a single failure yet, and it looks good and is light weight.

Works very well when coupled with the SOCOM RC2 and is still a pretty light package despite being a piston gun.

Why wouldn't you claim it to be top tier?  (IMHO, the made up "tier" ranking is 100% subjective and means nothing but allows people who have brand favorites to crow about their babies.  But anyway...)  Christensen is an excellent gun builder and one of the best gun makers out there.  They may be "boutique" from the sense that they aren't mass produced, but they certainly are no slouches.

Years ago, I conducted a direct comparison of a DI operated AR-15 and a piston operated AR-15, using the same Colt 16” 6920 barrel.

A.R.M.S. SIR with Ares GSR-35.












100 Yard Accuracy Comparison

Prior to installing the Ares GSR-35 unit on the 16” M4 barreled upper with the ARMS SIR free-float rail, I tested the accuracy of that upper in its direct impingement configuration from a bench-rest at 100 yards. Using hand-loaded 52 grain Sierra MatchKings, three 10-shot groups were obtained which had extreme spreads of: 1.59”, 1.55” and 1.73” for a 10-shot group average extreme spread of 1.62".

The three 10-shot groups were over-layed on each other using the RSI Shooting Lab software program to obtain a 30-round composite group. The mean radius of the composite group was 0.54”.

After the upper was converted to the GSR-35 piston system, accuracy testing continued in the same manner as above. Three 10-shot groups were obtained using the same load of 52 grain Sierra MatchKings. Those groups had extreme spreads of: 1.77”, 2.54” and 1.85” for an average 10-shot group extreme spread of 2.05". The mean radius of the 30-round composite group created from those three groups was 0.69”. This example demonstrates a decrease in accuracy of approximately 0.45 MOA for the ARES piston system.





……

I guess I'll pour gas on the fire.


Note these are just my own ideas about the piston DI debate in real world practical applications in that I see my AR as a defensive weapon.  For sport shooters it's a whole different mind set.  

A DI gun is still a piston in that the piston is located in the BCG as the bolt itself.  In theory the gas that would leave the firearm from a conventional Piston design above the barrel is now leaving through vented holes in the side of the BCG as soon as the BC moves rearward after being filled with gas via the DI gas tube.  I would speculated that there may be some amount of additional gas bleed off around the gas key/ tube interface .  What ever amount that is is the difference between cleaner and dirtier in a DI vs. Piston design.  What would normally be vented  at the piston block in total in a full piston design is now split between actual venting through the BCG vents and the gas key interface.  Is it enough to warrant a Piston over a DI system?  I would argue not really 90% of the time .  Today with some really reliable adj. gas blocks, DI systems can be made to use only enough gas that is needed to function without extra debris being set loose into the upper receiver.  Superlative's bleed off block would be just one example. The gas tube is another weakness in DI.  Stainless has been the old standby but it is still susceptible to rupture and over heating.  V7  now offers an inconel gas tube that is probably twice as durable but until enough are put through the ringer nothing is certain. However; since there will always be some amount of gas escaping from around the gas key, a DI all things being equal will run dirtier.  How much dirtier is quantitative on the amount of rds fired between cleanings and the types of ammo, suppressed or not?  Many variables.    Piston issues : Bolt tilt  The main contact point of the piston BCG is at the 6 O'clock position as the force is applied 180 degrees (12 O'clock) via an operation rod as it either hammers or pushes the BCG rearward. This has caused wear issues.  In a DI system the force reward is very linear, straight back as the bolt while locked acts as a stationary piston with gas imparting force in all 360 degrees  between it and the BCG. Pistons also have more moving mass to deal with, impart more vibration, weigh more and in some cases still need adjustments depending on suppress modes but it's the price to be paid for a piston system. Accuracy is not as inherent with a piston vs a DI due to the aforementioned aspect of mass and vibration.  Yes some pistons X brands will out shoot DI X brands but not on avg.

Real world?
After 500 rds ( over the normal combat load out) dirty is dirty, DI or Piston.  I've been told if one expels over 500 rds ( the whole team as an avg. ) odds are your are out of ammo and should call in air support or start evade and escape in that what ever operation you were trying to accomplish just got blown to shit. A slightly cleaner gun is not a priority any longer or if your not out of ammo your probably in a secure enough area to take 60 seconds and spray gun scrubber then break free to keep things moving unless the extra ammo you are using is because the guy next to you can't use it anymore ........... Most fire fights are won or lost in the 1st 5 min or less. At 3 30rd mags a minute that is some serious S#!T. After 3 mags your in clean up mode or rethinking the situation. If starting with a properly maintained weapon I see no difference between a DI and Piston reliability in this mythical 500 rd situation.

The DI vs Piston is an exercise in extreme possibilities.   I can get parts for a DI system anywhere.  To me an apocalyptic battle field in time would be littered with abandoned piston guns stripped of any useful DI parts based on too few of the system type and too many different piston designs. Pot luck pickings for piston parts. DI rules by sheer numbers.

My stuff is all DI and there is only one scenario where I would be concerned about DI which is the same one the USMC and Navy came up with ; "Over the beach".  The diameter of the gas tube just isn't big enough to allow rapid draining of water and can rupture from hydraulic action magnifying the PSI x 10. Again the inconel gas tube might mitigate some of this but a more sure way is to go piston and accept some trade offs. Enter HKM27/416.  A 1.75 MOA gun working 99.9% of the time is better than a .75 MOA gun working 93% of the time.  Just pulling #s out of the air folks.

The piston has it's place for those who feel it's warranted but many designs are here today and gone before tomorrow, even by big names i.e Ruger Sig etc..  For me a "having gun" to enjoy shooting and tinkering with sure but for something to be relied upon for both usage and keeping it operational in the real world like a post Katrina NOLA or worse, DI all the way.  Operating as a civi per the law should place one in a defensive posture.  Making a water based assault with a drowned weapon in an early morning raid is not a defensive posture but falling into a stream escaping a gang of ANTIFA thugs is too depending on your luck. Even when putting on another hat other than a civi I would still use DI.  Again, an exercise in extreme possibilities but possibilities no less.

This reminds me of the forward assist debate that still today some think as being extra Oz of useless "what if".  That is until I think of Kyle Rittenhouse's use of his FA  that saved his life while looking into the muzzle of Grosskrutz's 9mm.   No debate, the FA saved Kyle's life!   No debate, the piston too will save someones life as well in time.  It's about accessing the odds, all the odds and what you will use it for.

Philosophy of reliability big picture because I don't have an armorer with selves of support parts to hand it to when it's down.
Q: Is piston more reliable in all circumstance short term usage?  Yes.
Q: Can piston be maintained operational in all my possible long term circumstance  (parts)???????????? Maybe but most likely not.=NO

If it can't be kept operational it isn't reliable (big picture).


If I were to own a piston AR the HK M27 seems to have gotten it right despite a few early hiccups. It and only a few others have pass the test of time.  If they  (piston kits) are no longer available there is a reason.  

Side note: If you cross an AR15 with an AR180 you get a SIG MCX as far as I can tell ............................nothing wrong with that.

Counter point.  For your first point on cleaning and gas efficiency.  Keep in mind that most piston guns are considered self cleaning.  Not from the submerged in mud cleaning, but from the build up of carbon.  The action of the piston is generally such that it scrubs the carbon away during operations.  This is true of a DI in the interaction between the gas rings and the BCG, but not in the vents, gas key, gas tube etc.  So, in reality, piston systems generally don't need to be cleaned as frequently.  You are much more likely to develop gas inefficiencies in a DI gun than in a piston.  By introducing a "fix" you are putting DI guns into the realm of piston regarding your argument of unique/proprietary parts.  If you choose to add an adjustable gas block to your DI you are adding a part that is less common and less likely to be found "dropped on the battle field".  That kind of negates the argument.  Carrier tilt is not the panacea of problems that people seem to think it is.  Yes it's a thing, but it's no more detrimental to the operation of a gun than anything else.  As a counter, consider that the amount of metal fatigue in the BCG and bolt is greater on a DI gun because the mechanism runs hotter due to the injection of hot expanding gasses in the BCG.  You're more likely to see broken DI BCG parts than on a piston.  Carrier tilt is a significantly less important issue than metal fatigue.  Finally, no. Piston guns do NOT have more reciprocating mass.  This is a fundamental misunderstanding of short stroke piston operation.  A long stroke piston will have more reciprocating mass, but the short stroke version does not.  The BCG is the effectively the same as in a DI.  Op rods don't travel the full distance.  And piston systems do not by default weigh more.  The stuff you add to the handguard more than makes up the difference in weight which is on the level of grams of difference.  For that matter, slight variation in the barrel will have more impact on the weight than piston vs DI.

if you limit your shooting to 500 rounds, I would agree there's no point.  However, I have yet to clean any of my piston guns after thousands of rounds.  In particular, the Adams Arms system is thoroughly self cleaning.  I've taken them apart after years to see that with the exception of some carbon build up on the end of the gas port/tube, there is no functional need to clean them.  Aesthetics is different.  The Superlative system does have more carbon build up on the op rod, but nothing I'm concerned about.

Specifically regarding the statement about reliability.  Going back to the above, the removal of the heating/cooling cycling of the BCG and bolt reduces long term failure rates of the metal in a piston rifles.  It is more rare to have an op rod fail than to have a gas tube, gas rings, BCG etc fail.  So while the parts may not be as "readily available" the parts that are required to operate a piston gun are less likely to fail than the parts on a DI gun.  In the long term, I would guess than the failure rates are similar at worst.  Meaning there is no reliability benefit to DI over piston.  In long term rambo WROL situations, breaking anything will set you back.  And finding a replacement isn't likely to be a realistic fantasy regardless of the functional operation of the rifle.  Be it piston or DI, the only one that can keep it running is you.  So don't break it.  What rate of fire do you expect to have in that scenario?  You're more likely to run out of ammo before you wear and tear your rifle to the point of breaking it.  So DI/Piston is irrelevant.

Bottom line, it's personal preference.  There is not real actual tangible benefit either way.  DI/Piston = same same.  But it's not accurate to claim DI victory because reasons.  In every scenario you have mentioned, I would personally feel more confident with my pistons than my DIs.  Personal preference, not empirical data.

This maybe true for Civilian AR-15s that never fires full-auto.

Piston system in general is much widely adapted than DI System. It is the only type of operating system for machine guns.
Other than AR15/M16, Korean K1 is the only DI rifle that I can think of, Daewoo K2 has switched to long-stoke piston.
All other well know modern rifles are all short-stroke piston.

I don't think it make much difference when shooting semi auto on a bench.
I have both. My 308AR is DI, the lower receiver is not that much dirtier even when shooting suppressed.
BCG is much dirtier than piston after a few mag, but works fine.
DI is more accuracy for sure. Piston AR is heavier.
If you only have the budget to buy one rifle get the DI AR. Parts are easy to found.

So, no love for the AK pattern?  LOL.  AK pattern are all long stroke piston.

You say here today and gone before tmrw? Sig? You must be talking about the 516.  It is not gone. It's still running fairly strong. Mil/LE contracts for it. And you can still get them if you are Mil/LE. Sig is still making it along with the 716 piston. It's not going anywhere soon.  M27 is ok. Lots of Marines prefer M4. MARSOC don't even use the M27.  Just wanted to clear that up.  Your info was a little F'd up

Yes 516 Civi it is / was .....my bad

The Ruger is vapor now.

Have a light hearted laugh guys .  Feel free to switch it around.  

Interesting results. I would say a piston would always be inaccurate to some degree compared to a DI. Using some "thoughts" I would say as the bullet is passing the vent, gas is pushing a rod back while the bullet is still traveling down the bore. So the act of something moving is messing with the barrel's harmonic which in turn would degrade accuracy. Whereas a DI would simply be expelling gas down a tube, and the bcg needs to build up pressure before unlocking so the bullet is long gone by that time so there's no messing with the bullet itself.

However, I would say outside of lab conditions, you wouldn't really notice the inaccuracies of a piston unless you're doing some serious bench shooting.

See, on first blush, this sounds valid.  However, let's have a discussion about how long the round is actually in the barrel.  For standardization, let's talk about a 16" barrel.  And we'll use M193 ball for simplicity.  Muzzle velocity of that round in a 16" barrel is going to be somewhere around 3,050 fps.  Obviously it all depends on your rifle etc, etc, etc, etc...  Say midlength gas so the port is about 9" into the barrel.  

Total barrel time will be around 0.79ms.  Note that's not 0.79seconds.  0.00079 seconds.  The bullet is going to take roughly 0.59 milliseconds, 0.00059s to travel from chamber to gas port.  We'll ignore that section because that's the same on either system.  Dwell time is the amount of time it takes the bullet to travel from the gas port to the muzzle.  We can all agree that once it's left the muzzle, it doesn't matter what system got it there.  The dwell time of a 55gr bullet that has a muzzle velocity of typical M193 from a 16" barrel is .2 milliseconds, or 0.0002s.  I guess what I'm getting at here is, no.  The piston has not even started to move by the time bullet has left the muzzle.  It's still building pressure in the piston.  While I can appreciate the detail that Molon went to for his testing, we have to remember it's a sample size of one.  Which is interesting and informative, but not enough to make any conclusions.

I would be interested in seeing a conclusive comparison, but there are likely way too many variables to the comparison to make it feasible.  I would say it's valid to compare one manufacturer to another.  SO, what is the accuracy difference between a LWRC DI vs LWRC Piston?  That would be interesting.

ETA: I found an article where somewhere compared them with a range of ammo.  Not a very detailed study, but the end result was that there was negligible difference between the LWRC DI and Piston rifles.

I own and like both the same way, I own BCM/Daniel on DI site and couple of LWRC's on piston. But I am leaning towards less cleaning and more shooting and that is piston :)

Hi

Really great comments here and knowledge and I’m learning a lot.

I may have missed it but I’ve heard that piston guns reduce the exposure of carcinogens to the shooter.

Has there been any evidence to confirm this or studies ?

From the summary I’ve gained from reading, having a piston system on a conventional ar is unnecessary and may even bring some disadvantages.

However if having a reduced exposure to carcinogens and lead to the shooter. That is huge.

https://m.youtube.com/watch?v=Vg-sNfmsr44

This particular individual (great video by the way I may buy one) mentions a foreign study of reduced exposure to carcinogens

Only real benefit of a Piston especially Short Stroke i see is if you are running suppressed. Stoner made it DI for a reason
And for over 50yrs its been proven very reliable. I actually don't consider Piston ARs to be really
ARs anymore tbh.

You must consider the way you will run it. Piston is cleaner less wear on parts but SSGP has more kick than a DI.
Curiously I hear the LSGP by PWS has less than a DI.

My fear with a SS piston is if it broke in a shtf situation. I don't run a can so DI all the way. Also because that's how the man ES designed it.

Dupe

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