I was wondering what are the advantages and disadvantages of both.  They are primarily used for punching holes in paper.  I have a gas operated one (Doublestar)  that runs just fine.  I was thinking about adding another one to the collection in either a different caliber or maybe a piston operated one.  I saw the Remington ACR, way over priced, and it got me to thinking about the piston one.  So what are your thoughts?

I'd go to piston forum, you should find all the info you need.

gas vs diesel?

rotary vs piston?

Both "regular" and "piston" ARs are "gas operated".


What, exactly, are your needs and your budget?

I'll bite

piston runs cooler (and the benefit is)

A piston runs cleaner (I'd rather have a wet and dirty gun than a dry clean gun)

there no standerdized piston system ( once get a piston your married to that manufacturer, if something goes tits up then you have to go thru said company for a fix)

a piston is heavier and more moving parts

as you can tell I am not pro piston because with proper cleaning and maintaninice a gas gun will do anything a piston does  and if you look long term there are enough parts that will last for the rest of our lifetime

There is a six page thread about this on the first page right now.








 

Ares Defense GSR-35:  gas system retrofit range report.

Since I personally have never had any reliability issues with my direct gas impingement system AR-15s, my interest in a gas piston system is due mainly to curiosity.  (Isn’t that what killed the cat?)  The possibility of having to spend less time cleaning the bolt carrier group and upper receiver is also rather appealing.

Proponents of the gas piston system for the AR-15 claim it is more reliable than the direct impingement system, (which seems rather ironic since the gas piston system not only adds more parts to the overall system, but adds more moving parts as well.)  Opponents of the gas piston system state the AR-15 does not have reliability issues as long as proper cleaning and maintenance procedures are followed.  They also claim that the gas piston system causes a decrease in the accuracy of the AR-15.

There are a few different gas piston systems for AR-15s on the market, but most of them are proprietary affairs.  The allure of the Ares Defense GSR-35 is that it is user installable on your existing AR-15.  The GSR-35 kit comes with everything you need to convert your existing system to a piston system, including a set of modified M4 handguards.









You use the bolt from your existing system but remove the gas rings to run with the gas piston system.  I chose to use a Smith Enterprise chrome plated bolt as this would more easily show the fouling on the bolt for comparison.  I also chose to use a 16” HBAR for my testing and evaluation of the GSR-35 with the rationale that a heavy barrel would be the least likely profile to show a decrease in accuracy (if it actually did occur).  If a substantial decrease in accuracy was found using the gas piston system with an HBAR, there would be no point in even testing the system with a government profile or light-weight profile barrel.  The 16” HBAR I used for the conversion is a new chrome-moly barrel (not chrome lined) of uncertain pedigree.  (I think it was made from a Douglas blank, but it’s been sitting on my spare parts shelf for so long I’m not sure.)  The barrel is stamped as having a 5.56mm chamber and a 1:9” twist.


Upon removing the GSR-35 kit from the box, I was disappointed to see that the gas spigot was bent.  It was only slightly angled away from the gas cylinder, but you would think Ares Defense would hold a higher level of quality control on such a crucial piece of their system.

The Ares defense literature states their gas system is designed to work with “mil-spec” AR-15s. I decided to deviate a bit from the “mil-spec” by using a JP Enterprises adjustable gas block, for three reasons.

1.The JP gas block uses an extra long roll pin to secure the gas tube/gas spigot.  I thought this would help to mitigate any problems with the roll pin walking-out during firing, (as has been reported.)

2.The JP gas block is secured to the barrel using set screws.  I wanted to see if a set screwed gas block could withstand the additional forces applied to it by a gas piston system.

3.The JP gas block is adjustable.  I thought this might prove for some interesting experiments if the gas piston system proved useful.  (The gas adjustment was left “wide open” for this evaluation.)


Before installing the GSR-35 system on my carbine, I performed some informal accuracy testing from a distance of 50 yards using the direct impingement gas sytem. The forearm of the rifle was placed on a sandbag resting on a wobbly table.  There was no support for the butt-stock (nor for my elbows.)  The ammunition used was one of my hand-loads, using Sierra 52 grain MatchKings and VihtaVuori N135 powder.  As well as being an extremely accurate load, this load has functioned flawlessly in every AR-15 I own.

I fired three 10-shot groups in a row that measured, 0.558”, 0.408”, and .570” for an average group size of 0.512”.  The targets are pictured below.









Dennysguns has done a great job of describing the installation of the GSR-35, so I’ll not go into that here.  As I mentioned earlier, the gas spigot from my kit was slightly bent.  When installed on the carbine, this caused the gas cylinder to angle very slightly to the starboard side of the barrel.  This in turn caused some very slight binding at the juncture of the gas cylinder/gas piston/connecting link.


before conversion






after conversion











I began function testing of the newly converted carbine by loading and then firing a single round at a time from a magazine for the first ten rounds and then 3 rounds at a time for the next several magazines.  I experienced multiple failures of the bolt to lock back after the last round of the magazine was fired.  I don’t know if this was do to the slight binding of the mechanism I mentioned above, or if the system just needs a little “breaking in.”  After approximately 30 rounds the bolt began to lock back consistently.  There were no further malfunctions during testing.

I performed informal accuracy testing with the GSR-35 system installed on the carbine in the same manner as described above.  The three 10-shot groups that I fired from 50 yards measured, 0.677”, 0.501” and 0.879” for an average of 0.685”.  (targets pictured below)   This does demonstrate a decrease in accuracy (larger average group size) using the gas piston system compared to the direct impingement system, albeit a rather small one at approximately 0.35 minutes of angle with this 16” HBAR.









I fired a total of 90 rounds for this first test session of the GSR-35 and here is what the bolt and carrier looked like afterwards; (pictured below) not nearly as much fouling as would have been present with a direct impingement system.  Below is a picture of the inside of the upper hand-guard.  This is where the fouling goes with the gas piston system.  I was pleased to note that the JP gas block had not shifted forward at all during testing; nor was there any sign of rotational shifting.  Also, the roll pin securing the gas spigot held tight.













Over the next few months, I plan to fire 1,000 rounds of ammunition from the GSR-35 converted carbine without cleaning the gas system.  If there are no further malfunctions/problems, I will be doing some formal accuracy testing of the system from 100 yards.  If the results from that testing are acceptable, I’m going to repeat this course of testing with a GSR-35 installed on a 16” government profile barrel.

Here’s a pic of my GSR-35 carbine.







The highest part of the GSR-35 above the barrel on my carbine is the section on the gas cylinder just aft of the spigot.  It's the shiny spot on the gas cylinder with the red arrow pointing to it in the picture below.  The shiny spot was caused by the gas cylinder rubbing against the the underside of the upper handguard.









Here are some pics showing the differences between the two sets of handguards from the inside.















Today I fired 90 rounds through one of my direct impingement 16” uppers.  Here are some pics showing the Smith Enterprise bolt carrier group from that upper compared to the Ares GSR-35 bolt carrier group after firing 90 rounds.













I fired an additional 89 rounds through the Ares equipped carbine without a single malfunction.  When pulling the trigger on the 90th round, all I heard was the dreaded “click.”  I kept the carbine pointed downrange for 20 seconds and then examined the ejection port.  The bolt had failed to close completely on the round going into the chamber.  The bolt carrier still had 1/8" to 1/4" of forward travel to go.

I dropped the magazine and attempted to clear the chamber by pulling back on the charging handle, several times.  The action was locked up tight.  The lugs on the bolt had just barely started to turn and therefore were engaged with the lungs of the barrel extension.  I finally had to slam the buttstock against the ground while pulling on the charging handle to clear the weapon.

At that point, I chose to pause the testing until I could determine the cause of the malfunction.  Upon returning home, a quick visual inspection of the bolt carrier group, chamber and gas system revealed no obvious abnormalities.  I cycled a couple of dummy rounds through the action with no malfunctions.

I next turned my attention to the cartridge involved with the malfunction.  I dropped the cartridge into a JP Enterprises chamber gauge and the cause of the malfunction became painfully clear.  The cartridge failed to completely seat in the gauge.  Here is a pic of a cartridge properly seated in the chamber gauge and then one of the cartridge that caused the malfunction.








Scharch case.





I checked the headspace of the abnormal cartridge using an RCBS Precision Mic.  The headspace measured +0.008”.   That’s 8 thousandths of an inch longer than nominal headspace for the caliber.  Obviously the case had not been properly resized and that is why it failed to completely chamber in the Ares carbine.  I checked the headspace on the remaining 30 rounds that I had loaded for this test session and found two more cases that were not properly resized.

Since RVO is no longer a reliable source for once-fired and resized Lake City cases, I had decided to give the once-fired and resized cases from Scharch Mfg. a try.  That is the source of the cases that I had used in this test session.  This is the first time that I have ever had one of my hand-loads fail to chamber.  

Since I am confident that the malfunction was due to the ammunition and not the Ares gas system, testing will continue.  However, I will be chamber checking all the cases before heading to the range.  Here’s a pic of the bolt and carrier after a total of 180 rounds fired.






300 rounds


I fired another 120 rounds through the Ares equipped carbine today without a single malfunction.  (I used virgin Lake City brass for the handloads this time.)  Here are a couple of pics showing the bolt/carrier group and Ares gas system after a total of 300 rounds.














The inside of the lower receiver looks as if it has hardly been used.  The magazines are still remarkably clean also.

Just for kicks (and to prove a point) I fired some 75 grain A-MAX loaded rounds from the Ares upper from a distance of 25 yards.  The 10-shot group is pictured below.  Check out the key-holing!








400 rounds

I fired another 100 rounds through the Ares upper today without a single malfunction.  After a total of 400 rounds there has been no walking-out of the gas tube pin nor any movement of the JP Enterprises gas block.

Something I noticed today after doing some close range reflexive shooting drills is that the handguards seem to heat up quicker with the Ares system than with the DI system; particularly near the front of the handguards where the gas vents from the gas cylinder.  I may have to invest in an infrared thermometer to determine if this is actually the case.

Here is a pic of the bolt and carrier group after the accumulated 400 rounds of firing and below that a pic showing a comparison from the individual test sessions.












One of the attractive aspects of the Ares unit is that you can easily convert back to the DI system if you so desire with no adverse affect to your weapon (and then use the Ares unit to convert another DI system to the piston system if you like.)  It's too soon for me to make recomendations for the system.  For example, I perceived the handguards heating up quicker with the Ares system in my recent test session.  I don't know what affect, if any, this will have during extended shooting sessions or long term usage.  For my next test session I plan to put a lot more rounds downrange in one outing.

I will tell you this, I definitely like the decreased fouling in the receivers and bolt carrier group.   Also, while there may be a slight decrease in accuracy when using the Ares system, the accuracy is still more than adequate for my intended use.  In the last test session I fired the 10-shot group pictured below from 50 yards in an informal accuracy check, using handloaded 55 grain V-MAX bullets. The group measures 0.632”.  While not minute of angle, it is certainly good enough for government work.





550 rounds

I ran 150 rounds through the Ares upper today without a single malfunction for a total of 550 rounds.  The bolt is getting fouled at this point, but I can still see shiny chrome on it!





700 rounds

Another 150 rounds went through the Ares upper today with almost monotonous reliability.  That makes a total of 700 rounds through the Ares upper with no additional malfunctions since the 180th round.  Remember, that malfunction was due to the ammunition.

Using the Ares upper and 55 grain FMJ handloads, I fired a quick 10-shot group from 50 yards on a 300 yard E2 silhouette target reduced for 50 yards.  The results are pictured below, showing that while the Ares system might not produce MOA accuracy, it is certainly good enough for practical shooting situations even when using FMJ rounds.









Here's a pic of the bolt carrier group after 700 rounds.





800 rounds


800 rounds and counting with no further malfunctions to report.  200 rounds to go!








920 rounds

As I mentioned in a previous post, I wanted to let the Ares carbine sit idle for a few weeks without cleaning as part of the testing. I've read that even the gas piston on the AK can freeze-up and need a little kick start on the charging handle if the weapon is not fired for a while.

So, after letting the carbine sit for 4 weeks I headed back to the range.  I fired another 120 rounds through the upper without a single malfunction.  At this point, I decided to end the reliability testing.  I didn't think another 80 rounds would uncover any new problems that haven't been discovered in the last 920 rounds and I was eager to begin the formal accuracy testing.


Here's a pic of the bolt carrier group after 920 rounds.






Here's a compilation of bolt carrier group pics.







Here's a look at the bolt carrier group broken down.  Look at the tail on that bolt!  Did you ever see one so clean after firing 920 rounds?






Most of the fouling on the bolt and carrier was wet fouling and wiped off with a paper towel.  The rest came off with a little Break-Free and a nylon brush.  There was none of the typical cacked-on, hardened carbon fouling normally encountered when cleaning a direct impingement AR-15.

Here's a look at the Ares gas system after 920 rounds.










Here is the gas system broken down.






I was surprised as to the absence of cacked-on carbon fouling on the gas piston and in the gas cylinder.  Again, this fouling came off easily with some Break-Free and a nylon brush.  The gas spigot did have the typical hardened carbon fouling on it, but that would not normally be removed from the gas block for cleaning.  Here's a close up of the gas piston after 920 rounds.






Here's a pic showing the inside of the gas cylinder after 920 rounds (it was difficult to get a decent picture of the inside of the cylinder.)






Here's where a lot of the fouling from the Ares system ends up; deposited on the inside of the handguards.






On a totally unrelated note, here's what 1040 rounds of brass tracking on the case deflector look like.




As a recap of the reliability testing of the GSR-35, my unit experienced multiple failures of the bolt to lock back during the first 30 rounds.  After the initial 30 rounds, there was no further occurence of this malfunction.  Round number 90 failed to go into battery, but this was conclusively determined to have been caused by the ammunition.  No other malfunctions occured from that point on.


100 Yard Accuracy Test

Formal accuracy testing of the Ares carbine upper was done following my usual protocol.  Three 10-shot groups were fired from a distance of 100 yards off a concrete bench using front and rear bags to support the rifle.  A Leupold VARI-X III was used for sighting. The ammunition used was one of my standard handloads using Sierra’s 52 grain MatchKing.  Wind conditions were monitored using a “Wind Probe” (and it’s a good thing because it was quite windy during testing.)

I fired three 10-shot groups in a row from the Ares carbine with the extreme spread of the groups measuring:

1.04”
0.99”
1.20”

The average extreme spread for these three groups is 1.08”.  This is right on par with what my direct impingement 16” Colt HBARs will do.







The three 10-shot groups from the Ares upper were overlayed on each other using RSI Shooting Lab to form a 30-round composite group.  The mean radius for this composite group was 0.32”.  The composite mean radius of three 10-shot groups fired from my 16” Colt HBAR was also 0.32”!  Shown below is a comparison of the accuracy of the Ares upper with three different 16” Colt barrels.





This Is Rumor Control  

There have been a lot of rumors floating around the Internet (including some on this web-site) that gas-piston uppers possess some magical ability to propel a bullet “up to 200 fps faster" than a direct impingement upper with the same length barrel.  A simple application of science shows this is not the case.

I converted the Ares upper I’ve been using in the testing for this thread back to a direct impingement upper (16” barreled, carbine gas system upper.)  Using hand-loaded 69 grain Sierra MatchKings, I fired three 10-shot strings of this ammunition over my Oehler 35-P chronograph positioned 21 feet from the muzzle.  I let the barrel cool and then converted the DI upper back to a gas-piston upper using the Ares GSR-35 system.  Now, using the exact same barrel with the gas-piston conversion and the exact same lot of hand-loaded ammunition, I fired three more 10-shot strings over the chronograph, for a total of 30 rounds through each different gas system.

Here Are The Facts





As you can see from the table above, using 30-round samples for comparison, there is only a 7 fps difference between the direct impingement gas system and the Ares gas-piston system when using the same ammunition and the same 16” barrel.


Temperature- 63 degrees F
Humidity - 34%
Barometric pressure – 30.20
Elevation - 960 feet above sea level
Winds – variable
Skies – partly cloudy, Jupiter aligned with Mars


Government Profile Barrel Testing


For the next phase of testing of the Ares GSR-35 gas piston conversion, I wanted to install the unit on a government profile barrel.  I chose one of my Colt 6922 barrels as the test vehicle.  (The 6922 is a 16” M4 profile barrel with a 1:9” twist.)


Ares GSR-35 on Colt 6922





The first thing I noticed when installing the Ares unit on this barrel is that the gas spigot binds on a portion of the barrel near the front sight base.  This did not occur with the heavy barrel that I used in the original test.  One has to wonder what effect this might have on accuracy.


Arrow indicating area where gas spigot binds with barrel.




Original HBAR setup without any barrel binding.






I wanted to use a free float hand-guard with this set-up to avoid the problem of the unit binding on the Ares M4 hand-guards. Since Ares still has yet to release their “upside-down KAC” free-float rail designed for this unit, that was not an available option.  The only commercially available free-float hand-guard that I’m aware that will work with the Ares unit without requiring an alteration of the hand-guard is the A.R.M.S. SIR system.  I used the #50 slim-line, bi-level military version SIR.  The highest portion of the Ares unit clears the underside of the SIR rail by approximately 0.030”.

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".  While not the most accurate barrel I own, it’s still perfectly suitable for “government work.”

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 demonstrates a decrease in accuracy of approximately 0.45 MOA for the ARES system.








On the outside chance that the increase in group size was caused by the Ares configuration simply “not liking” the 52 grain MatckKing load, I fired an additional three 10-shot groups using a hand-load of 55 grain V-MAX bullets charged with a different powder.  Those groups had extreme spreads of 2.65”, 2.14” and 1.78” for an average 10-shot group extreme spread of 2.19" with a 30-round composite mean radius of 0.72”.   As I mentioned earlier on this page, one has to wonder if the Ares unit binding against the barrel is having a negative effect on accuracy.









Shortly after conducting the accuracy evaluation, the Ares upper began to have failures of the bolt to lock back after the last round in the magazine was fired.  Next, I noticed that the cases were being ejected less effectively to the point that they were just barely falling out of the upper receiver.  Finally, the upper completely failed to cycle.  I hand cycled a few rounds that fired but my Ares upper was now a one-shot AR-15!

After a quick examination of the gas piston system, I found that the gas tube pin had been sheared in half and the gas spigot had shifted slightly in the front sight base causing a misalignment with the gas port.  I haven’t used the small spring that is supposed to go in the gas tube channel from the period after my initial testing was completed since it was initially reported that the spring is not needed on the newer units, so this is the most likely cause of the shearing of the gas tube pin.  ARES later clarified that “the small spigot spring (is) essentially obsolete in most cases when the kit is installed in a standard carbine. If the sheet metal handguard cap is not used such as with some rail forends, then the spigot spring must be used to eliminate impact loading of the retaining roll-pin.









Here is a pic of the underside of the ARES M4 handguard showing the the channel worn into the heat sheild from rubbing against the gas cylinder.













Also interesting to note, is that when I installed  the ARMS SIR on the upper with the ARES system, there was a definite clearance between the gas cylinder and the underside of the SIR rail.  Yet, after firing there were clearly signs on the underside of the SIR rail that the ARES gas cylinder had been making contact with it during the firing cycle.

Question answered.  WOW, what great info and pics.  Thanks alot.

You should also be measuring bolt lug and barrel extension lug (headspace) wear if you are doing long term testing between the two systems, preferably with the same type (hardness) bolt.

For what it's worth, my Ares conversion failed spectacularly at ~150 rounds.

I know of absolutely no one who has had one go over 4000 rounds. Outside the manufacturer's claims, that is.

This is what you save over the life of a rifle if you shell out the cash for a pissedin upper. I'll just spend the $6 on the oil. I might even shell out more for some fancy space age gun oil, but pistons belong in commie guns and internal combustion engines.

and m1a's

No i never got the piston ar thing.  theres aks, g36's, ect.  if i want an ar its gonna be di....

The AR was designed around its DI system and pistons retrofits are less than optimal due to the geometry involved.  The step between the gas block and the receiver gas tube hole means there's no straight shot for the piston.  The bolt carrier is also designed to be pushed rearward from the center of the carrier (the bolt is the piston).  It does not have conventional guide rails to keep the carrier aligned when it is hit off-axis by a piston.

Well said.  I agree 100%.  Also, I was on a trip forward when the TF was pulling back their HK piston guns (which were fewer numbers anyway, than M4 variants) due to parts breaking/reliability issues SOME (read...NOT ALL issued HK piston weapons).  I think there has been some resolution to the issue, but I have not seen as many HK piston M4 type in 10.5" or 14.5" that I saw a year or two back.

THIS.

I don't like bringing old threads back to life, but these DI vs GP ones are always interesting.  Then there's the GP vs GP and which system be it OEM or retrofits.  

A lot of people have the mentality where if it costs more it must be better.  Well, most of us here know that's simply not the case.  I'm a tinkerer at heart, and once all these companies started coming out with retrofit kits I just had to try some out.  The ones that I've tried both replaced the gas block.  

I had the first version of CMMG's kit, and it did everything it promised.  My problem with it was the design.  It took a huge allen wrench to unscrew the piston plug.  Then you had to make sure you didn't lose the tiny washer and knock out the piston rod out from inside the receiver.  Now when it was up and running, everything was all gravy.  I'm sure it could go the recommended 1500 rounds before cleanings; however, I was a marine and just had to clean it after ever range outing.  Cleaning became way more complicated than your standard DI system.  I had to use a 9mm bore brush to clean out the piston cup and apply anti seize the piston plug thread every time I put it back on.  It just felt wrong that a 9mm bore brush and a ratchet w/ allen key drive became a normal part of my rifle cleaning kit.  The piston return spring also had to be replaced every 7500 rounds.  I never came close to that before I sold the upper, but this kit took something simple and made it complicated.  Of course CMMG knew of these issues and updated their design in the Summer of '09.  They overhauled the gas block and piston plug making it much more user friendly.  They did keep the same piston design that needs a 9mm bore brush to clean it.

A system I did end up liking and currently use is the Adams Arms retrofit.  Even compared to OEM piston uppers like LWRC, this is the most simple system I've come across.  The entire guts of the piston system come out the front of the gas block with a 1/4 turn after pushing in a detent.  There are also not many small parts that come off it.  It's essentially two pieces, the plug and drive rod.  The problem I have with this system is also in the design of the return mechanism for the drive rod.  It limiting because the standard barrel nut (or one of equal thickness) must be used.  Regardless, I am very satisfied with this system.  It's very easy to clean, and it's so nice not having to take a brush to anything in the bolt carrier group.  According to their demo video "the tolerances between the sleeve and plug allows the drive rod to float freely keeping the harmonics from being transferred to the barrel and making the Adams Arms piston system the most accurate on the market."  Regardless if this is true or not, I didn't notice a decline in accuracy comparing the same model barrel.  I ordered two White Oak Armament 18" SPR barrels with headspaced bolts.  One went on my friend's DI setup with a 10x40 Bushnell Elite 3200, and one went on my Adams Arms rifle with a Trijicon Accupoint 3-9x40.  Both were able to shoot sub moa groups with factory PMC .223, Remington .223, Winchester 5.56, Federal XM193, and Hornady 68gr BTHP loads.  However, we were only doing groups of 3 every 30 minutes from cold bore on sand bags.

I remember reading a review Jay Leno wrote about the Tesla electric sports car.  He talked about innovation.  One thing he mentioned was it had to do everything it's predecessors did and take more than just a step forward.  He used rotary engines as an example of a great idea, but not true innovation in terms of internal combustion engines.  In my opinion AR piston systems are similar.  They aren't the end all be all of Stoner's original design, but more of a luxury for the lazy.  That being said, the gas piston isn't a replacement for direct impingement nor is it an upgrade.  This is why I haven't replaced all my DI AR's with GP or retrofits.  I will admit one thing though... my GP AR's see tons more range time than my DI ones, but that's because I'm lazy.

Osprey..no carrier tilt,no springs,no screws or valves 3 main parts 2 pins 1 pin is solid to pin in the cyclinder to the FSB and a carrier the fewest parts of any piston system I believe and adds 4 ounces more than a gas tube while eliminating the tube and bolt rings.....100% reliabilty,durability and spare parts support.Get used of pistons everyone because the interim upgrades requested for the M4 by the army specify piston operation.Not everyone comes from experience and shooting backgrounds or maintains their weapons as they are taught so something that will run with or without lube in any climate making it practically imune to neglect that soldiers can dish out is a good thing.Cooler and cleaner and will run lubed or not = way more reliabilty like it or not DI operation has been around a long time AKA the ljungman..

So if you look at the worlds most sucessfull small arms systems the 1 common denominator is gas operated pistons...theres no doubt that piston operated weapons will run reliably longer with less maintinence than DI operation.DI running sucessful depends on good support and maintinence and some times you dont get either in a fast paced combat zone.What if you run out of lube or cant maintain the weapon for a few days...in some climates thats a weapon killer for an M16 series rifle.So by 2013 when the upgrades are done DI will be dead...flame onis it a luxury for the lazy maybe but the facts are some soldiers are lazy and with an M16 lazy can cost you or worse someone else their life.Not every grunt maintains weapons like more experienced or specialized troops so a weapon that is as soldier proof as can be is whats needed.

I love when weight is brought up in the piston systems...with all the crap people put on their carbines to make them M14 heavy wich was not Stoners intention of a light weight due to revolutionary materials used in construction..4 or 5 onces added at least with the Osprey is really of no concern a rail weighs 2 or 3 times as much.

Are you saying you think parts rifle are unreliable?