AR: Direct Impingement or Gas Piston?

[Aquila]

Zak, there is FAR less carbon buildup on a gas piston carbine vs a direct-impingement one...

The other advantages listed are also very clear.

People resist change; it's normal but it does not have to be so.

Does anyone think a Direct-Impingement conversion kit for the AK would make it MORE reliable? I don't think so.

 

[Aquila]

Zak, there is FAR less carbon buildup on a gas piston carbine vs a direct-impingement one...

The rest:

The other advantages listed are very clear for all to see.

People resist change; it's normal but it does not have to be so.

To Anyone:

Does anyone think a Direct-Impingement conversion kit for the AK would make it MORE reliable? I don't think so.

 

[Coronach]

Zak isn't saying that there's no carbon fouling, or that GP guns don't deposit less carbon. He's saying it doesn't matter, as carbon fouling in general is not the source of malfunctions in ARs, and that it doesn't happen specifically in the CHAMBER (as you say), since the DI system is not dumping gas there.

Does anyone think a Direct-Impingement conversion kit for the AK would make it MORE reliable? I don't think so.

This is, once again, not the question being asked by the OP. The question is not whether or not DI is better that GP, or vice versa. The question is whether or not a GP retrofit into a DI design is better than just a straight DI design.

Mike

 

[30 cal slob]

i'm a fan of the gas piston setup.

partly because i am a lazy bastard.

"cleaning" a carbine after shooting one all day (or, more recently, all night) following a class boils down to swabbing the bore and maybe tossing a little lube on the BCG.

 

[Jaws]

The AR15 type rifles are good and the design has many good things. But as much as i understand people like to deffend their favrite toy, DI is not one of the good things of the design. Is one of the bad things. Do we really have to jump and deffend even the bad parts of a product just because is our favrite?

There's a good reason we instal oil filters in an engine: to keep the carbon and residue from the burned gas to mess up our oil. We try to keep out what the Direct impingement is putting into the mechanism.
Why do you think the we change the oil in the engine every 5000 miles or less, and we change the oil in transmission only once in a blue moon? Because in the transmission the oil doesn't come in contact with so much of the heat and carbon resulted from the burned gas.

The only reason people are deffending the DI is because they feel the need to deffend the AR15 no matter what.

Even AR15 can be clearly improved with a well designed piston system. Look no further then HK416.
Ask those special forces that had to give back their HK416's for M4's what they think.

 

[Coronach]

The only reason people are deffending the DI is because they feel the need to deffend the AR15 no matter what.

No. Actually, I'm a fairly reluctant AR15 supporter, and I'm not a huge fan. I do, however, own several and I have yet to have reliability issues. I do think GP designs, in general, are a better way to go when designing a weapon system. However, I am completely unconvinced that retrofitting a piston into the AR makes the weapon any more durable or reliable than an AR running direct gas. Even the vaunted HK416 is having some rather vexing problems.

Mike

 

[Zak Smith]

False and specious arguments against DI and for GP do not help progress, they hinder it through bad information. Suffice to say: carbon fouling does not cause malfunctions in properly-built AR-15's (even after thousands of rounds without cleaning); GP conversions do not have five decades of experience rooting out their idiosyncrasies; external debris such as sand will eventually cause both GP and DI AR-15s to malfunction due to the BCG and lug design.

Thank you, Coronach, for correctly explaining my position while I was at lunch.

-z

 

[MCMXI]

I am completely unconvinced that retrofitting a piston into the AR makes the weapon any more durable or reliable than an AR running direct gas.

If done right, from a purely theoretical viewpoint, why wouldn't it be more reliable? However, without real world, side by side comparisons we'll never know so it's left up to theories and conjecture ... in other words, the status quo at THR ... but still interesting nonetheless.

Despite the numerous comments that carbon fouling doesn't cause jams or reliability problems, I wonder how many here have had FTF situations with light primer strikes due to carbon fouling in the BCG. I have DI and GP ARs (and like them both) but my life doesn't depend on my choice yet so I have the luxury of time to decide which system is more "reliable".

Wait-- OH NO! Someone on the internet doesn't like my rifle!

I could care less too, but I'm here to learn so if someone has a good point to make I'm all ears. Arrogant people don't learn anything ... they just tell everyone else how they THINK it is.

Anyway, Mad Ogre's last paragraph was particularly interesting to me ...

"I wrote this piece a long time ago... It has garnered me more flak than anything else I have ever written. It is linked to many discussion forums all over the net where people who disagree with my opinion elect to voice it through personal insults and asking questions to their forum - but not to me. That's fine. I don't consider myself to be taken to task by zit faced, roll-playing, counter-strike addicted pussies."

 

[H2O MAN]

I prefer GP rifles that have five decades of experience rooting out their idiosyncrasies


I have no problem with a quality built DI AR.

Oops!

gvnwst, Thanks!


.

 

[gvnwst]

I have no problem with a quality built DI AR.

I think that is what you meant... Either that, or you are using some intresting grammar.

 

[RP88]

Yes, carbon fouling is reduced in the chamber by a gas piston. But I'm still waiting for someone to answer my other question: how does any of that matter when sand gets dumped onto the weapon? How would any of the newer piston guns that seemingly would melt in the desert conditions handle the sand?

 

[gga357]

Is there another fire arm with a DI design other than AR10 and AR15? Honestly a question I am curious about.

 

[Deer Hunter]

Isn't the Hakim DI?

 

[Pulse]

Is there another fire arm with a DI design other than AR10 and AR15? Honestly a question I am curious about.

there is one gun from the late 1930's or early 1940's that used a DI system, Swedish design if i am not mistaken and there is the Accuracy International AI-AS50 that uses a DI system.

thats about it as far as i know.

 

[gga357]

Found it it does mention a gas tube:

The Automatgevär m/42 (Ag m/42) (outside of Sweden commonly known as the AG42, AG-42 or Ljungman) is a Swedish semi-automatic rifle in limited use by the Swedish Army from 1942 until the 1960s. wikipedia

 

[Coronach]

If done right, from a purely theoretical viewpoint, why wouldn't it be more reliable?

Define "done right." By this you could mean something along the line of "replacing the gas tube and gas key with a high-quality piston and op rod", or you could mean "replacing the gas tube and gas key with a high-quality piston and op rod, and enlarging the passage in the upper for the piston/op rod, and redesigning the BCG and upper reciever to better deal with the change in impulse from using the piston in lieu of DI". After a certain amount of fiddling you no longer have an AR with a GP, you have...something else. That something else is not necessarily bad, but it certainly is no longer just a GP AR.

To answer your other question, I hinted at it above. Piston uppers impart a different type of pressure on the BCG. Since the BCG was designed to handle the impulse from direct gas, you can end up with odd wear issues, parts breakage, and jams from the bolt/BCG moving in a different way from what it was intended. Also consider that any piston/op-rod design has to pass through the really really narrow space designed for the gas tube, which results in an op rod that is a lot thinner than you really may desire. This is especially true of true "drop in" units, though even dedicated GP uppers still have to contend with the relatively confined area above the chamber, unless they want to really redesign how everything fits together in the rifle, and then we're getting into the whole "is it still an AR?" debate.

Mike

 

[Joe Demko]

French MAS-49 was direct impingement. In production from 1951 to 1965; in service until 1980 when the FAMAS replaced it. Some MAS-49's are still in use in various parts of the world. It was a somewhat quirky gun, but the DI system is not noted as a problem.

 

[iamkris]

For what it's worth, Small Arms Review just did a 2 part interview with Reed Knight. While he didn't defend DI systems in general, he gave pretty poor view...from a mechanical viewpoint...for a piston AR.

* High torque to the bolt / BCG
* Uneven wear to reciever due to "tipping" BCG
* Uneven wear to bolts
* Cracking bolt lugs
* Cracking carrier keys
* Cracking key screws

The biggest problem is that the system wasn't originally designed for it. Not saying piston systems are bad in all rifles...just that they aren't "all that" for ARs.

He did say that pistons are better in ARs for barrels shorter than 14.5"...mostly because it is difficult to design a reliable DI gas system for shorty ARs.

 

[Coronach]

The biggest problem is that the system wasn't originally designed for it. Not saying piston systems are bad in all rifles...just that they aren't "all that" for ARs.

Yep. Everyone thinks that if you add a piston it makes it better. The problem is the 95% of the system that is left after you switch out the DI for the GP.

Mike

 

[MCMXI]

But I'm still waiting for someone to answer my other question: how does any of that matter when sand gets dumped onto the weapon? How would any of the newer piston guns that seemingly would melt in the desert conditions handle the sand?

Oil in a firearm is a double-edged sword providing lubrication but attracting (and holding on to) dust, dirt, sand etc. Picatinny is currently testing different lubricious coatings on M249 parts for this very reason. POF gas piston uppers are designed to run with zero oil in the gas block, piston or connecting rod and the BCG, bolt, charging handle etc are all coated to reduce the amount of oil necessary for reliable operation. As tight as the tolerances in the AR can be, I bet that an oil-free action would be more reliable in a sandy/dusty environment.

iamkris, does Knight's Armament offer a gas piston AR? The POF BCG doesn't have a separate carrier key (it's integral to the carrier) so it doesn't have any carrier key screws either. As for Mr. Knight's other points, are they merely his opinion or are his comments based on empirical data? If his company offered DI and GP systems I'd be more inclined to see his comments as objective. Bushmaster offers both systems so what's their position on this?

Define "done right." By this you could mean something along the line of "replacing the gas tube and gas key with a high-quality piston and op rod", or you could mean "replacing the gas tube and gas key with a high-quality piston and op rod, and enlarging the passage in the upper for the piston/op rod, and redesigning the BCG and upper reciever to better deal with the change in impulse from using the piston in lieu of DI". After a certain amount of fiddling you no longer have an AR with a GP, you have...something else. That something else is not necessarily bad, but it certainly is no longer just a GP AR.

I think you make an excellent point ... a gas piston conversion kit "done right" would most likely be cost prohibitive compared to buying a gas piston upper to start with. By "done right" I mean something along the lines of the POF system which would necessitate a new BCG.

 

[marktx]

Isn't the Hakim DI?

The Hakim being a direct descendant Ljungman is direct impingement as is it's little brother the Rasheed. The French MAS 44,49,49/56 series of rifles are also examples of the type.

I have only shot my Hakim and Rasheed a little bit but they seem like simple, reliable rifles. I doubt they would be improved with a piston of any type.

The MAS44,49,49/56 series of rifles is more familiar as I have shot mine quite a bit. Brick sh-t houses envy their sturdy construction and so far mine have been 100% reliable. They are extremely simple and well put together weapons and are one of my favorite semi automatic rifles. I don't shoot corrosive ammo in mine (original ammo is rather hard to find and I don't like to clean) but the MAS would be very easy to clean up after.

 

[iamkris]

does Knight's Armament offer a gas piston AR? The POF BCG doesn't have a separate carrier key (it's integral to the carrier) so it doesn't have any carrier key screws either. As for Mr. Knight's other points, are they merely his opinion or are his comments based on empirical data? If his company offered DI and GP systems I'd be more inclined to see his comments as objective.

From the March 2009 SAR issue:

"Knight: ...I should probably qualify my statements here this way: In our testing over the last two years, we have fired about 1.3 million rounds of 5.56mm. In that process, we have come up with some things that we think improve the M4 rifle in its baseline design. We've also built some piston driven uppers that drop on the basic lower. If there is anything that I am really convinced of, it's that you should not go and think that the piston driven upper system, based on the AR-15 chassis, is some type of a major improvement, because in fact, we have found it not to be..."

It is an understatement to say that Reed Knight knows the AR platform better than 99.99999999% of people living on the planet. He probably knows what he is talking about.

Me...I have no dog in this hunt. Just reporting what I read.

 

[MCMXI]

It is an understatement to say that Reed Knight knows the AR platform better than 99.99999999% of people living on the planet. He probably knows what he is talking about.

Too many 9's there since that'd make Mr. Knight 0.68 of a person (half the man he used to be) but I get your point ... I just don't agree with it. There are hundreds if not thousands of people that know virtually all there is to know about the AR platform. It's been around long enough and it's just an assembly of pieces, it's not cold fusion for God's sake. Mr. Knight's opinion isn't a statement of fact despite his obvious expertise. It's just his opinion based on his business plan and what he knows or believes to be true at the time, which is based on the tests that he ran, under the conditions that he chose or was presented with. No one is infallible and time will tell if gas piston uppers are are here to stay. Fortunately, there's a tough proving ground readily available so we'll see how it all turns out. For now, I'm happy that I have DI and GP versions to play with ... and I like 'em both ...

 

[iamkris]

Mr. Knight's opinion isn't a statement of fact despite his obvious expertise.

Nor are the opinions of 99.999999% of the people on the internet. (based on internet penetration of 23.8 % and a global population of 6.7B)

Like I said, I have no dog in this hunt...I just tend to trust the opinions of folks that have more demonstrated knowledge like Zak or Mike.

 

[Aquila]

* High torque to the bolt / BCG
* Uneven wear to reciever due to "tipping" BCG
* Uneven wear to bolts
* Cracking bolt lugs
* Cracking carrier keys
* Cracking key screws

[1] Shot peened, MPI'd AISI 8620 alloy of the dimensions of the bolt carrier tells me that the high torque isn't that big of an issue - for the bolt carrier.

[2] I've yet to hear reports of actual uneven wear but I don't doubt that there exists evidence. I'm a trained engineer of 20 years and it's obvious that any eccentric loading will result in assymetric wear patterns. Is it an issue? I doubt it.

[3] The AR-15 already has uneven loading on the bolt lugs. Adding high temperatures to the mechanical cycling can only worsen the problem, no? Yes, I'm correct here.

[4] LMT has a one-piece bolt carrier thus no shearing of keys or bolts.

Just like the venerable Porsche 911; fantastic sports car that it is and lust object of mine since 1974. If I were to start from a clean sheet of paper two things my auto would NOT have: a flat six powerplant and it would not reside outside the wheelbase. Apparently, Porsche, too, recognizes that the game has moved on so we have their ultimate supercar being a V-engine design and the layout being mid-engine. The other cars - the Boxter and Cayman are mid-engine but retain the flat six for cost saving measures and marketing purposes.

I've no dying love of the AR-15 as it could be more robust and less finicky as a design. However, when it's running properly what outperforms it? Very little, if anything. That does not mean that it could not be made better.

Give me a gas-piston rifle if it's semiautomatic. Period. Just like the rest of the world.