There was an interview with C. Reed Knight in a fairly recent issue of Small Arms Review,  in which Mr. Knight explains why the AR platform is not suitable for a gas piston system and
should be kept as a direct impingement gas system.


I was told of this interview and the reasoning but to be perfectly honest I have not had a chance to read it for myself as of yet.


But, the physics of it are not too hard to understand.


With the direct gas impingement system,  when gas goes through the gas tube and into the bolt carrier via the snorkel,  the gas pressure forces the bolt FORWARD, holding it
securely in place,  locked in the barrel extension,  while the pressure behind the gas rings forces the bolt carrier backwards.   The bolt is pushed forwards by gas, and the bolt
carrier is pushed backwards by gas.     The cam action rotates the bolt out of battery and the inertia imparted to the bolt carrier pulls the bolt out of the barrel extension.
The pressure applied on the carrier is almost entirely aligned with the direction of travel of the carrier.  The carrier gets pushed straight back,  excluding some relatively
minor torqueing forces applied via the gas stream through the snorkel.    The carrier action is very linear, without any significant lateral forces applied to the upper receiver
or carrier.


With a gas piston system,    there is no supplemental pressure burst against the rear of the bolt to hold it securely in battery while the pressure curve drops off,  which may or
may not be significant,  but more importantly,  the driving impulse on the bolt carrier is off -axis by a respectable amount,  well outside the outer diameter of the carrier, which
makes the carrier torque toward the bottom of the upper receiver,  causing rapid premature wear, and in some cases, a minor misalignment between the upper receiver bore
and the buffer tube bore can become a point at which the bolt, being driven back with a considerable downward torque force on its rear edge,  snags and takes some damage
with every shot.    Bolt carriers that are used in piston guns should have beefed up bearing surfaces on the lower section of them and the lower rear of the carrier should be
given a good, well polished ramp cut.

Knight also says that bolt breakage is considerably higher in piston ARs than in direct impingment ARs.        The reason is apparently due to the very sudden impact delivered to
the bolt and carrier by the gas piston when driven by the gas stream,   compared to a more moderate push given to those parts by a direct impingement gas system which
has a definite pressure curve profile to it, much different from the virtually "square wave"  pressure curve delivered via a piston system.


CJ

40+ years of direct impingement gas system.  I say that is good for me.  

Heads down!  OP, does anyone know where you live?  Can you still delete your profile?  Good luck, mate!

Makes perfect sense if you ask me.

I've read similar feedback about the cam pin area of the bolt potentially being a weak link in the current piston conversions. No matter what your opinion on pistons vs DI, step back a minute and acknowledge that mechanical devices involve wear and tear. Eventually something will break in either system, so buy the best you can, and hope for the best.

The funny thing about that interview with Knight is that Colt defense said the same exact thing.  What do know, Colt just released their new piston driven carbine.  I think Knight was saying that because he doesn't have one one the market.
I abused my LMT piston AR pretty bad.  Never had anything break.

I still think that a properly deisgned piston system is hands down better, just maybe not in the Stoner/AR platform. I've read where the
weak link in the original Stoner design was the bolt anyway (mechanically speaking.... ignoring temp spikes and fouling in the reciever).

That said, I think that some of the current gen 2 or 3 piston systems on the market are just as reliable as any DI system... so IMO,
it's just Knight and Colt's attempt at saying that their current designs are the best b/c if they said otherwise, they'd have to offer
"otherwise" or look like fools.

-ZA

I was convinced long before reading that.

That's a good read though.

I agree. Why try to fix something that's not broken in the first place? I don't really see the point in spending all the extra money on something that, so far, only keeps the gun

cleaner and has not been proven to add any significant reliability to the platform.

I'm sticking with DI.... BUT! The forward pressure on the bolt doesn't occur until the bullet has already passed the gas port.

I have a PWS kit one one of my uppers and I like it.  So there...    

Funny.... because those with lots of range time on ARs... once purchasing a Piston system and hitting the range a few times... they are sold on the piston...

I got a POF Upper... and At first I was kinda leary... but All aspects considered... and the side by side testing/ comparison... I am IN LOVE with my POF and could never go back to DI....

Pretty much

I don't claim to be an expert and if someone else comes to the conclusion that a piston gun is what they want they are welcome to them.

Seems to me that a regular old AR has a pretty good battle record with a bit of basic cleaning,care and lube I will put my trust in something I know and can expect to get standard perts for

AMEN!

After several years in the grunts and 2 tours in the desert I can say I never had an issue with any M16 or M4 I was issued.

You only need to do three things to keep an AR reliable. Muzzle cap, Closed Ejection Port Cover and mag in the well.

Gas Piston is just another fad.

Yep ARs should be DI



leave the pistons to piston guns

who in their right mind would want a AR that was reliable as the AK? dem piston guns are no damn good

I'll be glad to put my trust in the opinion of C. Reed Knight.   His deuces don't stink.


I might get thousands and thousands of rounds of reliable shooting out of a gas piston upper, but then again, I may encounter severe upper receiver
and buffer tube bore wear and broken bolts.   I don't see the need to experiment in that regard.    

I take it for granted that the AR platform is a gun that needs occasional cleaning...just like any other firearm.   Even an AK will eventually start having problems if
you NEVER clean it, you know.

I've seen how clean an AR will stay if the gas system is blocked off.  I did that myself once.  (I had the gas tube in upside down. )  Sure, it was as clean as you
could ever want a gun to be that's been fired since its last cleaning,  but the charging handle doesn't lend itself to many operations in the same way that a
bolt gun or lever gun's action does.   My fingers were sore pretty fast,  manually working the action,  and I always had to do a tap-rack drill after every shot,
so it was way more trouble than the clean action was worth.


CJ

Opinions are like a-holes. everybody has one. I love my new LMT gas piston just as much as I love my DI CMMG. Nothing's wrong with a little variety.

I'll run my Bush GP against ANY tube gun. 5000 rounds, I see less wear than a tube gun, runs alot cooler, easier to clean, no o-rings, don't need gas rings, cycles great . I've run all kinds of different ammo no issues.
The only draw back is purchase cost! I don't know about the other pistion systems but I'm happier than a pig in poop with mine. Cooler and cleaner is the advantage.

Indeed, as evidenced by the fact that your ownly contribution to the discussion is that you like your new toy.

I'm happy for you.

When it comes to the AR15 platform I prefer gas impingement because that's how it was designed run.  I personally believe that piston conversions change the way an AR runs a creates another whole set of issues.

Also, I've personally fired well over 2000 rounds without cleaning and I'm convinced that I can't carry enough ammo for the cleaning issue to make a difference.

I do think that there are some folks who have legitimate need for a piston AR but in most cases it would probably be better use a different gun designed to run with a piston instead of converting it over.

Regarding the carrier tilt issue, I believe one of the members at M4C designed a really neat bufffer that fixes this issue.  If I were running a piston AR I'd definitely have one of those his buffers.

tag

Ok, fine. I've owned several DI AR's and done more than my share of time in the sandbox to know how well they perform. I never once had a malfunction with any of my DI rifles that wasn't magazine related. I absolutely LOVE DI AR's and I would trust my life with one any day. I don't have nearly as many rounds through my LMT to make fair comparison, but it's been 100% reliable as well. It's also cooler, easier to clean, and shoots softer than my DI middy. It hasn't shown any signs of abnormal wear, nor is it less accurate than any DI rifle that I have shot. Thanks.

What piston gun did Colt just release?  I'd like to get one!

I was busting up laughing when my buddy kept taking out his LWRC bolt carrier after shooting a few rounds. "See that dude? Still cool to the touch! I can touch it."

It's called the Colt APC which stands for Advanced Piston Carbine.

Link:   http://www.defensereview.com/modules.php?name=News&file=article&sid=1184

If you think Knights doesnt have piston guns think again, Both the knights PDW and Stoner LMG are piston operated.

ARs should be kept DI. If you want a piston go to another platform that was designed for piston operation from the start.

The people that shoot the most in the US Mil...and who can have whatever long gun they choose, have chosen an HK 416.  

These are the same folks that were going through a bolt and barrel a week in a training cycle on DI guns...

That says a lot about properly made piston guns.

OK I'll play.

I do not know all of the specifics of all the AR type piston set-ups out there.

The idealist piston set-up in the AR platform would be one similar to the ZM with the majority of the mass in the operating/guide rod. Imagine combining the Sig 556 operating system in the AR platform. Shouldn't be too hard.

My $.02, YMMV, IMO, etc., blah, yada...

I like the benefits a GP design brings to the table.

Maybe someday I will have one from LMT.

Parts for DI systems, while becoming expensive are still widely available.

When the inevitable GP part breaks or is lost,  most offerings have just one source for replacement.

Hats off to the early adopters, but I think I'll wait till greater market penetration has occurred.

Just my 2 cents.

that is funny because its not true, colt has no  piston rifle, the 6940 is DI    and thr APC is a hybrid  tat still has a DI operation in it.

I dont understand  why people will trust something 2 or 3 years old more than something that has been around and worked for over 40. Piston may well be a  good system, but I wont buy one for at least 10 years of seeing a piston AR15 type used in combat

if colt is not making a piston rifle what the heck is the colt APC

My M4 took a shit on me twice, once when it didnt matter (qualification range; barrel came loose) and once when it really did count on mission in Iraq (bent gas rings).  Any one sticking with DI should add a set of new gas rings to there kit/repair parts.  7 cents that stopped my rifle when i needed it.  Not cool.  I have a piston gun now, POF design.  No gas rings, but there may be some truth to premature wear of the upper time will tell.

DI is good but keep spare gas rings/bolt on hand

Pistons rule.

We donn need no steeeking peestons!  

as long as we keeps our weepons berry klean, which is easy to do in a desert or in mud








 

Correct, but I think his point was that the bolt is being pushed forward while the carrier is pushed backward at first (until the cam has rotated the bolt out of battery), which would reduce the stress on the lugs.

I'm not questioning your experience here, but that's the first I've ever heard about bent gas rings. How do you bend the gas rings?

Less is more. Stoner knew what he was doing.

Stoner designed a 20 inch gun with a rifle length gas tube.

The M-4 is a very different beast.  The MK 18 even more so...

I don't think anyone is complaining about 20 systems being non functional...but they are like muskets.  You need a carbine and when you get into short barrels with DI systems, you get short service lives and extraction problems.  It is the realiy of a DI system.

For what it was designed to do in a 20 inch gun, DI is great...but we aren't using 20 inch guns.  

We? Im curious now. Marines what do they use mostly M16 or m4s?  Army units?
IS there a break down?
Btw Im a 20 inch barrel fan.

the model T Ford is a great car there is no reason to drive anything else

Around 30,000 M4s and 110,000 A2/A4s with all the A2s other than training units going away.

That's funny you call it a musket because 20 inches years ago use to be considered short. I agree though I always thought the 20 inch M16 was more reliable then the M4. I wonder if Colt's piston system was just them catching on to it just becaues it's a fad or is it legitimate?

The model T was phased out because technology improved.

I'm really not sure the gas-piston is a technological improvement - if I wanted to argue, I guess I could mention that the gas piston design was around BEFORE the AR.


None of that's really relevant, though - the question is simply this: is the gas-piston design better than DI? If not, why bother? If so, is the improvement enough to worry with?

Gas Pistons are the new future of the Black Rifle. Sticking to DI is like continuing to drive a 72 ford mustang and not step up to a Shelby GT500. DI is a cheap and DIRTY way to make a rifle. I took my M6A2 and blew through 200rds in under 2 minutes without one failure PERIOD.

Looks like this is another thread debating the placement of the piston since the DI uses a piston also.

lol, do you even own a DI gun?  You think going 200 rounds is some sort of accomplishment?