[Alex45ACP]

Are there any disadvantages to the gas piston uppers compared to the current DI style?

[Chris Rhines]

From what I've seen of them, they are less reliable than the conventional AR.

- Chris

[Alex45ACP]

Huh? I thought the whole point was that they were supposed to be more reliable :confused:

[Dave Rishar]

Disadvantages?

Needless complication, primarily. The current system has worked for 40 years. Gas pistons were not a secret when Stoner designed the weapon. He did not choose DI accidently, or simply to be contrary.

The reliability issue is often touted. I remain unconvinced that one is noticably better than the other in this department. DI probably gets dirtier per shot. Most AR's don't seem to mind it. It's important to remember that when most argue the inherant reliability of piston designs, they're (perhaps inadvertantly) referring to arms like the AK or M1/M14 that use an op rod as well and probably gain their reliability more due to having such a large reciprocating mass than anything, something most (if not all) AR piston uppers don't seem to share. Many piston-operated weapons that lack such a mass don't do nearly as well. Just having a piston in there doesn't necessarily fix anything.

Sometimes I get the impression that piston uppers are more about selling stuff than improving stuff, but that's just my take on it. Others feel differently.

I smell a 9mm vs. .45 debate coming up. ;)

[Chris Rhines]

Huh? I thought the whole point was that they were supposed to be more reliable Difference between intention and effect, I guess...

- Chris

[SigfanUSAF]

9mm v. 45ACP debate? Gas-piston v. DI? What about 5.56mm v. 7.62mm?
Gimme an M1A in my hands with a 1911A1 in my holster, and I'm happy as can be.
Anyone can modify a firearm. A genius improves one. I'm the former, but agree with Mr. Rishar. Go with what has worked for 40+ years.
How many TRUE improvements have been made on the 1911, for example? Mods to feed modern loads, mods for comfort, heck, in most cases we revert back to the original flat mainspring housing.
Sorry to have diverted, but I firmly believe Mr. Stoner knew what he was doing.

[saltydog]

I smell a 9mm vs. .45 debate coming up.

Nah, I double tap both my 9 and my 45 so I could care less.:D

[Langenator]

The advantages are that your not dumping as much heat and carbon into the upper receiver and moving parts. If you're not doing sustained heavy firing, that probably doesn't matter a whole lot to you.

Personally, I'm thinking of buying one for the simple reason that I do like ARs, but I hate cleaning the carbon off of basically everything.

[DMK]

Huh? I thought the whole point was that they were supposed to be more reliable It's basically a solution in search of a problem.

I love when folks tout it as new tech. :D

[Gewehr98]

It's not a bad idea. Those that I've seen could probably stand some refinement, but I'd wager 2nd generation AR15 gas piston uppers will be around the corner soon.

Never did like the M16 in my 20+ year military career, but I cleaned each one I was issued like there was no tomorrow.

[Dave Rishar]

In my own experience, M16A3's undergoing heavy firing (defined, in my case, to be a rate of fire that melts the handguard and/or destroys the gas tube) don't choke on the carbon or heat. The weapon is still operating properly at the point where one can no longer hold it without gloves. Once it cools down, though -- well, that is a different story. I don't see a piston helping with that sort of heat however. The barrel is often trashed after this and the operating system won't fix that.

As for the cleaning issue, I will accept that. I've never had to clean a piston AR but I can't imagine it being any dirtier than a DI. It's not that bad, though; my preferred method is to scrape the worst of the carbon off with a stripper clip, soak everything down well with oil or solvent, and start scrubbing. There are only a few spots where the carbon really builds up and even then, it's not that big of a deal.

It's worth repeating that the gas piston (and direct impingement, for that matter, which was ~50 years old when Stoner considered it) are not new solutions to anything. They're merely different roads leading to the same destination. For the M16/AR-15, and what it's meant to do, it's my belief that DI is just fine. For something else, other rules apply.

Let us return to the OP's question: assuming an equal build quality, are there any disadvantages to a piston upper from a purely theoretical standpoint? I submit the following:

1. Unnecessary complication.
2. Added weight.
3. Increased/altered recoil impulse.
4. Increased parts count.
5. Fouling concerns with gas system.*
6. Cleaning concerns with gas system. **

* You know and I know that a properly designed and manufactured gas cylinder and piston are essentially self-cleaning; however, consider every time that someone has posted on here or somewhere else about how their gas piston isn't moving freely under gravity due to fouling and such, and think about how many DI AR users have complained that their gas tube was blocked. There's something to be said for the AR's gas tube in this regard.

** Clean rightside-up if the gas system is on top and upside-down if it's underneath, right? So says the manual, because we don't want solvents and oil getting in there and dieseling or coking later. Now, it's theoretically possible that the same thing can happen in a DI but really, does it?

There are probably some things that I'm missing. More additions are welcomed. Let's answer the OP's question.

[IZinterrogator]

Huh? I thought the whole point was that they were supposed to be more reliable They are more reliable in barrel lengths like 11.5". Since most of us don't own an AR that short thanks to the NFA, it's a moot point.

That being said, I wouldn't mind having one to try out, but only if it said Colt on the side. ;)

[Gewehr98]

I wouldn't mind having one to try out, but only if it said Colt on the side.

McNamara and his cronies didn't want anybody messing with the existing design, so the Army skipped Colts' next iteration: (Picture credits go to Badger Arms)

http://www.thehighroad.org/attachment.php?s=&postid=718289

As for Stoner's adaptation of the Direct Impingement gas system being the epitome of the design, I heartily disagree. My 1942 vintage AG-42B Ljungman is also direct impingement, and considerably cleaner in operation than any AR-15 and M16 I've ever owned or been issued. Note to gun designers who absolutely must use direct impingement operation - don't route combustion products through the bolt carrier, mmmkay?

[Lebben-B]

I own a POF-USA upper (It was my present to myself after I got back from Iraq.) A couple of points:

Reliability - I have experienced superior reliability with the POF over my DI AR and my issue rifles. Through 3K over the last seven months, I've had nary a problem with the POF. The first 1000 rounds were fired with no lube and no cleaning, just to see how reliable the system was. The POF performed to spec with zero issues.

Maintenance - In reality, this is a zero sum game. Yes, the receiver halves, bolt group and chamber stay cleaner on my POF. But the carbon has to go somewhere and that somewhere is the piston system (piston, piston sleeve, gas plug, and tappet.) Other than that, normal -10 maintenance applies.

Recoil - The recoil impulse is different. Not harder or more pronounced, just different.

Complexity - The POF system replaces a gas tube, and bolt carrier group with a gas plug, piston, sleeve, tappet and modified BCG. (The LW system uses a tappet, spring and a gas cup, IIRC) Yes, it adds parts but those parts actually increase the reliability of the rifle by keeping waste heat and material from reaching the BCG and receiver. As previously stated, this really becomes an advantage with shorter barrelled uppers and while firing with a supression system since there's very little issue with gas blowback and dwell time.

Weight - This is an issue, at least with the POF system. The POF piston system makes the AR noticably nose heavy. It doesn't require herculean effort to put the sights on target, but the system is more weighty than a comparable AR/M16 series/M4.

So what are the disadvantages to a gas piston system? Weight and a different maintenance emphasis. Some posters in other threads have pointed out that the US has used a DI rifle for over 40 years. And then go on to state (usually) that gas piston systems are an answer in search of a question or somesuch pithy comment. Consider this: If the DI system is the end all/be all of gas operating systems, why did it languish as a firearms footnote until Stoner made it work acceptably well? DI is deceptively simple. Yes, it requires fewer working parts. But it demands that those parts be finely made with tight tolerances in order to achieve a proper gas seal.

Mike

[LoadedDrum]

I do not get the complication complaint either.

For instance, with the POF, you lose 3 gas rings which are wear items in a DI AR. When was the last time you wore out a gas piston? You also lose the staked on carrier key and requisite bolts. Three less things to get loose and marr your upper with. The parts that a piston adds do not wear (except for the spring in the LW).

Okay, the gas system is adjustable. All that means is set your gas system for the lowest setting that will reliably cycle your most commonly shot ammo and enjoy less recoil and wear on your AR. When you do run into that bad batch of Wolf or what ever, just crank it up one. Not that complicated.

[chieftain]

There is/was a good reason no other successful fighting weapon has Gas impingement.

Someone said it earlier, don't S**T where you eat.

Problems with the extractor, piston rings, and wear factors on the bolt carrier.

Some one else said, "it has worked for 40 years." Mainly when no one was shooting at our troops.

There are reasons the Army wanted to replace the AR system. one word RELIABILITY. I believe the only reason we didn't is the cost. Needed the money for other stuff.

With the right bullets we can get by with the caliber. I prefer a 'better' one, like one of the newer proposed 6.5/6.8 type used.

I policed up Dead Marines in 1967 with their brand new M16-E1 model rifles were in parts around them. Let's just say it left an impression.

In 2003 we had a whole support unit was virtually wiped out because their rifles couldn't function. Dirty, no doubt. (think Jessica Lynch) Never did hear of that happening in one of our other wars, where the AR system wasn't used.

There was a reason. 35 years later, and American troops dying for the same reason. their M16 would not function in combat.

Looks like we may have to go back to the Frozen Chosin soon too. I ain't so sure that the Matty Mattel will cut it under the winter conditions there either.

I don't know who has the best piston weapon, POV, Robinson, Hk, SIG, FN, Armalite but I bet one of them has one that is acceptably reliable right now. I doubt I know everyone that is making a piston impingement system.

And let's not forget HK's earlier Delayed roller locking system either. It was known for it's reliability.

go figure.

Fred

[SigfanUSAF]

Here is a fact. Any firearm is going to require some level of cleaning. I've had to stop shooting my #4 mk 1 after hundreds of consecutive rounds due to the fact the bolt wouldn't close. The chamber became too gummed up. This is especially true for any semi-auto. I was a 2A551B MOS, so I never experienced a firefight situation with my A2, but always took the time to clean it thoroughly after every use (try it while flying in a 130 in incliment weather!!). As with any machined object with moving parts, any buildup of anything in between parts may cause a problem. The tight tolerances of the AR system are its biggist downfall, IMHO. The Hk roller locked rifles, AKs, etc have much more forgiving clearance, which allows them to be inherently more reliable. Considering the original design of the direct gas operated AR/M16, its incredibly reliable for what it is.

[Bartholomew Roberts]

For instance, with the POF, you lose 3 gas rings which are wear items in a DI AR.

Is the POF bolt a proprietary bolt different from the normal AR15 bolt? Because in a normal AR15 bolt, the gas rings also bear the load as the bolt moves in the carrier. This is why the bolt in a LW gas piston system must have gas rings to function even though there is no longer any gas being routed to it.

[Lebben-B]

Is the POF bolt a proprietary bolt different from the normal AR15 bolt? Because in a normal AR15 bolt, the gas rings also bear the load as the bolt moves in the carrier. This is why the bolt in a LW gas piston system must have gas rings to function even though there is no longer any gas being routed to it.

The POF bolt is a standard AR bolt, sans gas rings. IIRC, LW left the gas rings on his system to ensure the bolt stayed stable as it moved inside the carrier.

Mike

[Bartholomew Roberts]

The POF bolt is a standard AR bolt, sans gas rings. IIRC, LW left the gas rings on his system to ensure the bolt stayed stable as it moved inside the carrier.

Hmmm, that is interesting - have you noticed any signs of increased wear on the lugs or bolt bounce compared to a regular AR15 bolt?

Makes me wonder how critical that part is to proper function if one gas piston manufacturer is not using gas rings at all and another is. Even a direct impingement AR can run fine on a single gas ring... I wonder how much of a role keeping the bolt stable plays in reliability or longevity?

[Lebben-B]

Hmmm, that is interesting - have you noticed any signs of increased wear on the lugs or bolt bounce compared to a regular AR15 bolt?

Makes me wonder how critical that part is to proper function if one gas piston manufacturer is not using gas rings at all and another is. Even a direct impingement AR can run fine on a single gas ring... I wonder how much of a role keeping the bolt stable plays in reliability or longevity?
I haven't had any increased wear on the bolt lugs nor is there any more wobble or bounce in the bolt than in a standard AR. That said, the bolt moves freely back and forth inside the carrier. I think Paul left the rings there as a redundant system, "just in case."

It's also interesting to note that the POF system doesn't use a return spring for the tappet, as the LW system does.

Mike

[DMK]

FWIW, I've had both a FAL and SKS choke on carbon built up in the gas tube (obviously from the lack of cleaning, I used to think the piston and tube didn't need cleaning). The gas piston design doesn't solve that problem. It just moves the problem somewhere else.

[Exposure]

I got to play with a 9.25 inch POF upper in full auto a couple of weeks ago.

I really liked it. Seems like a well built, great performing upper to me. Granted my time on it was far to brief to make any kind of assesment but as a whole I thought it was a really interesting take on the whole AR-15/M-16 platform.

It sure did attract a lot of attention at the range and functioned very reliably through magazine after magazine.

Throws quite a fireball too with the 9.25 inch barrel!

[whofan]

I have the Leitner-Wise rifle. I got is in mid-December of '05. After about 2500-3000 rounds through it here are my observations.

Recoil impulse is definitely different. I can't really explain it, but it seems to be quicker, if that makes sense.

Reliability. I have never been in a position to really test that. Though, I will say this. The bolt, carrier, chamber, etc stay much cleaner and cooler. Cleaning is just as simple as wiping the parts off.

There are about 4 extra moving parts. The weight is negligible. I can't subjectively feel any difference when compared to similar configured DI rifles.

Maintenance of the piston system itself. None, so far. It does blow some junk on the front sight base and on the last .5 inch of the top rail. These are not moving parts so I don't worry about it much. The front sight wipes off easily. The inside of the rail does get a very small amount of carbon built up then it seems to not add any more. I no longer worry about it.

BTW, the piston parts look nothing like the monstrous things in that Colt drawing above. Most people who look at my rifle don't even know that it is a piston system until I show them. In fact, only one guy figured it out and that was from the name on the side.

[Gewehr98]

FWIW, I've had both a FAL and SKS choke on carbon built up in the gas tube (obviously from the lack of cleaning, I used to think the piston and tube didn't need cleaning). The gas piston design doesn't solve that problem. It just moves the problem somewhere else.

I've done that same test with an AK or two, and left the gas tubes and pistons exceptionally cruddy with nary a problem. Especially with some "filthy" H335 handloads that weren't the cleanest burning choices in ammo. Of course, cleanup later was considerably easier than an AR/M16. ;)

[Alex45ACP]

Weight - This is an issue, at least with the POF system. The POF piston system makes the AR noticably nose heavy. It doesn't require herculean effort to put the sights on target, but the system is more weighty than a comparable AR/M16 series/M4.

What barrel length is yours? Also what barrel profile does POF use on their rifles? If it's the heavy barrel, is it possible to switch it with a lighter one?

[chieftain]

First off,

If you don't clean a fighting weapon at the first opportunity after every, I say again, every use you are an idiot.

I don't care what system or weapon your using/carrying. I did two tour with the 3rd Marine Division in Vietnam, and then was a DI. I wasn't the only 'hat' that when 'nuts' when I found a dirty weapon, not just on the drill field, but in the FMF too.

I have in fact talked with DI's of today, in the Corps. Any folks planning to join todays Marine Corps, don't leave your weapons dirty. And please call me if you plan to use the "Sergeant, I was testing my rifle to see how dirty it can get and still work." routine. I figure I can make a lot of money on the audio, and probably the video too. When I was a 'hat' you could have.

I love the excuse of "testing" my weapon. I will Guarantee that every weapon ever made will sooner or later jam related to being dirty. If it is a fighting weapon, what are you waiting for?

The specious argument that all weapons get dirty, and all will jam is just that.

There are those who have used and or possess the gas impingement systems who have no experience with mechanically operated systems. They tend to place emotional connections to the gas system. There are some theoretical advantages of the gas system. But, for the same reason no major weapons system other than the AR has ever used it, it is operationally unsound.

The only reason we wound up with it was because it was a political decision, not a weapons based or combat decision. What other country let it's Air Force select it's primary infantry weapon????

We are still trying to get the system right. Hell, we are still trying to find the bullet rifle twist combination that works best. We are still paying a lot in man-hours to keep the rifle functional. And still looking for a weapon to solve the problems/weaknesses of the AR system.

The best thing the AR system brought to the table, IMNSHO is the concept of modularity.

I have always believed we needed more than one caliber/type of primary weapon for our infantry. For terrain like Afghanistan I would prefer a 7.62 NATO type rifle. In Iraq the Lighter smaller, shorter for Urban combat is better suited. I don't believe any one cartridge/rifle system can do that.

Build a lower that can do both, and just swap tops. Make it quick change barrel capable. Now we have a very versatile system. The 6.5 Grendal and 6.8 SPC are still just compromises, one do all cartridge.

Maybe one of them can pull it off. I doubt it.

Most likely a combination, of 7.62 NATO Top with a 5.56 NATO top. Go back to the 55 grain for the 5.56 which worked just fine for stopping power inside 100-150 yards, and stick to the 150gr or go up to 168gr for the 7.62 for when you want to reach out and touch someone.

My only point in the end is that the mechanical systems will win out in the end. Why, they do every thing that a impingement system needs to do, better and ultimately more reliably.

Go figure.

Fred

[Bartholomew Roberts]

chieftain, much of your information is based on incomplete information. To use just one example, let's discuss the unit you mentioned.

First, the weapons malfunctioned the very first time they were fired. If the problem that makes direct impingement unacceptable is that it dumps carbon back into the bolt/chamber area, then how is direct impingement to blame?

Second, the same unit had numerous crew served weapons malfunction - M249, M240, M2. None of these are direct impingement. Given the wide acceptance and positive reports in the theater from other troops (as well as few reports of these types of incidents), how is it that they are happy with their direct impingement rifles?

The direct impingement is a perfectly reliable system. The problem comes when you design a working direct impingement system based around particular specs (20" rifle barrel with 13" gas port and 55gr ammo using a specific powder) and then change all three specifications and expect the system to work with the same parts you originally designed. Gas piston or direct impingement, there is a limit to how far you can modify a self-loading rifle from its original design without affecting reliability.

The reason a gas piston has come into vogue has nothing to do with reliability of the M16 or M4. Let's say that the M4 is a horrible weapon and has a Mean Rounds Between Stoppage that is 1/3 that of the Swedish AK5 (5.56mm AK gas piston design). This would give the M4 a MRBS of 1,166 In what situation do you envision a soldier firing 1,166 rounds of ammo through his rifle without having an opportunity to clean it? Even if we do reach this horror scenario, what is the result? Clear the stoppage and go another 1,166 rounds. On top of that, the M4 does considerably better than 1,166 MRBS.

The whole gas piston AR thing is driven entirely by units with a requirement for 10.5" barrels, suppressors and liberal use of full-auto. If you don't have those requirements, then the direct gas v. gas piston debate is just Internet fodder. Even then, you probably could design a firearm that would work reliably as a direct gas system - you just would have a hard time doing it using parts from a 20" AR15 system.

[Langenator]

Most likely a combination, of 7.62 NATO Top with a 5.56 NATO top.

You're pretty much describing the FN SCAR-L/H combo, except it does it by replacing the barrel and lower-you can't just replace the upper on a standard AR platform, because the mag wells are different sizes due the the differing dimensions of the two rounds.

The SCAR upper receiver/rail platform is longer than necessary to accomodate just the 5.56 (this also makes it a bit heavier), but this allows it to accomodate the 7.62 as well.

[chieftain]

Bartholomew:

The carbon deposits are a major SECONDARY problem. The primary problem is heat.

Heat is what destroys the extractor, alters the metallurgy of several components and springs. Throw in the carbon deposits, often literally being cooked on to components, and you have your recipe for jams.

I do agree with your analysis of changing the design dynamics. But that is one of the secondary gains of a mechanical system. It is easier, relative statement to be sure, to change for the dynamics you mentioned.

Any military weapon must keep heat and carbon build up away from the action as much as possible, not direct it into the action.

Military weapons with high rates of fire, particularly automatic weapons will have increased number of problems when the heat and carbon are dumped into the action.

AS to some of the newer designed weapons, lets get a lower profile. The flat top AR's are fine. Dump the handles and such. Increasing the eye above bore height only increases the number of troops that will be hit in the head during a fire fight when our guys are behind cover. That is an absolute.

In a stand up fight the height of the sights don't matter, in a knock down drag out fire fight, where everyone is on the deck, and behind or in what ever cover is available or dug, signature of the shooter matters.

Fred

[Bartholomew Roberts]

Heat is what destroys the extractor, alters the metallurgy of several components and springs.

I wasn't aware of heat failure of the extractor being an issue in the AR15? There were issues with the extractor slipping off the empty brass in the M4; but this was due to the shorter cycle time and increased residual pressure in the chamber when extraction began. Heat is an issue but only because the hotter the chamber becomes, the longer it takes brass to shrink away from the chamber walls. I can't really see how direct gas affects that particular heat issue.

Could you also elaborate on what metallurgical components are being altered by heat? My knowledge here is pretty limited so I can't figure out how the stainless steel gas tube can direct enough heat back into the upper receiver to alter the metallurgy of any part of the upper receiver in normal operation.

But that is one of the secondary gains of a mechanical system. It is easier, relative statement to be sure, to change for the dynamics you mentioned.

Depends on the change you are making to the system... for short carbines, gas piston looks like it solves more problems than it creates.

In a stand up fight the height of the sights don't matter, in a knock down drag out fire fight, where everyone is on the deck, and behind or in what ever cover is available or dug, signature of the shooter matters.

One reason the AR has such light, easy to control recoil and minimal muzzle flip (even in heavier calibers) is because of the straight line stock that allows the recoil to come straight back. The problem is that a straight line stock also means that you have to raise the sights or they are unusable. The flattop ARs you mention using Aimpoints and Eotechs actually have a height over bore pretty close to iron sights (about 2.5").

In order to lower the sights, you need an angle stock like the FAL and you need to replace the buffer and buffer spring assembly of the current stock as well. Whatever you might think about direct impingement design, the straight line stock has clearly been a winner in the design wars - L85, XM8/G36, FN SCAR, Robinson XCR, Robinson M96, Steyr AUG, etc.

You really see the advantages of that system when you are trying to shoot while moving or transition between multiple targets quickly. The AR will tolerate a little sloppiness in form and still be manageable. The FAL requires a lot closer attention to form if you want to do well. Personally, I think the extra 2" of exposure is a fair trade for the easier managed recoil; but if you don't there are lots of older systems out there that have a lower height over bore.

[ctdonath]

Tacking a suppressor on can increase the DI blowback to uncomfortable levels. Firing a suppressed short M4 full-auto (modest bursts) gives one a nosefull of unpleasantness. May not stop the mechanisms from working, but making the operator gasp for air doesn't help.

[Lebben-B]

What barrel length is yours? Also what barrel profile does POF use on their rifles? If it's the heavy barrel, is it possible to switch it with a lighter one?

I have a 16" barrel with a Vortec flash supressor. The barrel is fluted and is offered with continuous fluting or a gap in the fluting for the addition of a grenade launcher or other underslung attachment. If I'm not mistaken, POF also offers standard profile barrels as well. But the heavy nose isn't due to the barrel, it's from the gas plug, piston and sleeve. The heavy nose has one positive side effect - the muzzle settles quicker shot to shot, particularly when firing rapidly. FWIW, as Whofan posted, the LW system doesn't suffer from nose heaviness because of it's use of a gas cup.

Mike

[Foxtrot427]

Which would you rather clean a piston or the...everything else?

[Dave Rishar]

Again, from personal experience, the handguards caught fire before anything failed because of heat. This was with a newer FN-manufactured A3, last year, and M193 ammunition. I cannot supply a lot number (it was over a year ago, like I said) but it was from LC. There was some damage evident after the weapon cooled down but it was still functioning fine when it caught fire. It's worth noting that the primary heat problem was in the vicinity of the barrel -- I do not believe that a gas piston would have helped things here.

I'm somewhat ashamed to admit that I've seen this repeated a number of times. The usual result is a fractured gas tube and/or a "droopy" barrel after cooldown. The weapons were still operating properly well into cook-off territory.

As far as the height of the sights, I'm not buying that one. We're talking about an inch or two. Show me hard statistics to support this and I'll consider it further.

In regards to the other issues, Mr. Roberts has addressed the salient points far better than I can.

From a strictly designer's point of view, venting combustion byproducts into the action is a bad idea. Locking the bolt to the receiver (rather than the barrel) with a mere two lugs is also considered by most to be a bad idea; as history has shown us, however, there have been some very successful designs that did just this.

Coincidence?

[chieftain]

Bartholomew :

Many M4/AR operators use an 'O' ring over the extractor.

One reason the little 'O' ring is needed because the extractor spring goes south rather quickly and loses it's temper because of heat.

The tendency of the extractor and bolt lugs to be brittle. Tendency of the Military AR's to have an occasional Cook off post fire fight.

And of course overall durability. The military weapons don't last too long particularly with SPECOPS because of wear. Now those boys will wear any weapon out. They just wear the M4/AR systems faster than any of the others.

Here is a paragraph from JDW:

From Janes Small Weapons Review. RE:

The comparison of the SCAR to the M4/M16.

[Sidelight: The powder fouling wasn't the only major problem with the early M16's. When you want to hear the real story let me know. I was there.

Chieftain]

The SCAR tappet gas system is a departure from that of the M4A1 and resolves a problem that has plagued the AR family since its original design - fouling and particulate matter being blown directly back into the receiver. Not only does this make for a very 'dirty' gun that requires fairly intensive maintenance, but with the wrong powder excessive fouling can actually cause stoppages.

This was the case when the original M16A1s were fielded. The US Army specified the wrong powder and in the crucible of Vietnam, many soldiers and Marines lost their lives when their rifles ceased to function due to excessive fouling. This problem was resolved nearly 40 years ago, but the M16 rifle and M4 carbine still get a blast of hot gases and fouling with every shot fired.

Nearly as bad, the hot gases and fouling directed into the AR's receiver cause the weapons to rapidly heat up under full automatic-fire conditions. This is not much of an issue with semi-automatic only guns, but with Special Forces carbines that are frequently fired extensively on full automatic, heating is critical.


This has been an issue for quite a while. Luckily until recently (Iraq II) we haven't been in situations where our troops were in day in day out combat for extended periods of time. Or unable to get to armorer services easily.

These are facts, not opinions. A well designed piston system will answer some of these problems. Probably why Colt is coming out with one too. They see the handwriting on the wall.

Fred

[Lebben-B]

It's worth noting that the primary heat problem was in the vicinity of the barrel -- I do not believe that a gas piston would have helped things here.

Actually, it probably would have. As the receiver heats up, the barrel attached to the upper receiver would heat up as well from transferance. It wouldn't be the primary cause of this fire you claim, but it would be a contributing factor.
...Locking the bolt to the receiver (rather than the barrel) with a mere two lugs is also considered by most to be a bad idea; as history has shown us, however, there have been some very successful designs that did just this.

A two lug lock up never dumped heat and carbon into the receiver. I've used the M16/M4 professionally for over two decades in every sort of environment imaginable. Yes, DI "works." So why change? Even the most ardent AR fan will admit the AK is a much more robust weapon. A gas piston system simply adds more robustness to the AR.

[Bartholomew Roberts]

Many M4/AR operators use an 'O' ring over the extractor.
One reason the little 'O' ring is needed because the extractor spring goes south rather quickly and loses it's temper because of heat.

Hmmm, I hadn't heard the claim that the O-ring was needed because the extractor spring goes south due to heat and I am skeptical of that claim. If this were true, then we should see the same problem in the M16 series since it has the same gas system; but the problem never occured until the M4 was developed.

A much more likely explanation is that because the M4 tries to extract faster than the M16 while there is still a higher residual pressure in the chamber and the brass is still obturated, the original M16 extractor often slipped over the rim because the brass was still stuck to the chamber walls. The problem gets worse as the barrel gets hot - not because the spring has weakened its temper; but because it takes even longer for the brass to cool and shrink away from the chamber wall as the barrel gets hot (and with less mass the barrel heats up faster).

Note that you can fix this problem without using an O-ring at all just by going to an M4 extractor spring and black insert.

The tendency of the extractor and bolt lugs to be brittle.

I've never heard of an incident of a brittle extractor. There are plenty of reports of bolt lugs snapping off in M4 carbines; but this again is probably not related to heat. If heat were an issue, we would see the same thing in the M16.

I believe a more likely explanation would be the fact that internal pressures on the bolt are increased 1.5x in the carbine and some design issues on the lugs. For one, the lugs are undercut as a design compromise where they meet the extractor in order to have a wider extractor claw and distribute the force over more area (to prevent ripping rims off brass). Not a problem in the original design; but when you increase the force 1.5x - a problem. The second issue is the way the lugs are cut dramatically increases the stress where they meet the bolt. If they were just radiused there (like pretty much all of the enhanced bolt alternatives for the M4 did), it reduces the stress dramatically but requires new barrel extensions in many cases.

Finally, the M4 can continue to run with missing lugs on the bolt. The main danger that the broken lugs pose is not the actual absence of the lugs; but the tiny bits of broken lug floating around in the internall waiting to get jammed into the workings.

Tendency of the Military AR's to have an occasional Cook off post fire fight.

In the M4s, cook-off can happen after as few as 180 rounds fired back to back on full-auto. Cook off is a function of chamber temperature (about 600F). How would a gas piston weapon with the same characteristics have a lower chamber temperature than a direct gas weapon?

And of course overall durability. The military weapons don't last too long particularly with SPECOPS because of wear. Now those boys will wear any weapon out. They just wear the M4/AR systems faster than any of the others.

No question a gas piston would be more durable than an M4. Heck, you could make the M4 more durable DI just by moving the gas port forward two inches so that port pressure, internal bolt pressure, cyclic, dwell and all those other issues were more inline with the rifle. That is the nature of logistics - a change that makes perfect sense when you are only changing ten rifles starts to be downright backwards when you have to apply it to 1,000 rifles.

Nearly as bad, the hot gases and fouling directed into the AR's receiver cause the weapons to rapidly heat up under full automatic-fire conditions. This is not much of an issue with semi-automatic only guns, but with Special Forces carbines that are frequently fired extensively on full automatic, heating is critical.

No question that heat is bad for the internal parts of a rifle.... but this argument makes no sense to me. How does DI cause a weapon to heat up more rapidly than a regular gas piston? Even if we are just referring to the internal parts, the one critical part that is actually cited as the problem is the bolt - and the bolt is already in direct contact with the combustion gases in the chamber whether you use a DI or gas piston.