DI vs piston for AR

tiriaq said:
Comparing the AG42B, Hakim, MAS 49 with the AR is misleading.
The first three are DI.
The AR is not. Its operating system is different.
It uses a ducted gas operating system; gas is ducted back into the bolt carrier, where there is an expansion chamber. Gas expanding in the chamber forces the bolt carrier and bolt apart. This gives rearward motion to the carrier and rotates the bolt to unlock. This is why there are piston rings on the bolt, and gas escape ports in the carrier. The AR system can contribute to fouling inside the carrier, not in the chamber.
In the first three, the gas tube forms a bit of a nozzle which engages the bolt carrier. The bolt carrier is blown back off the gas tube extension.
If the passageway inside the AR bolt carrier key were blocked, so that gas could not enter the expansion chamber inside the carrier, the system would be like that of the AG, Hakim and MAS. But would it even work?
A major advantage of the AR ducted gas system over the AG and MAS is that the operating system is able to compensate somewhat for ammunition variation. The Hakim has a valve, the AG and MAS don't. Lack of adjustement or compensation makes a design rather ammunition sensitive.
Click to expand...

That was an impressive post....:cheers:...

Starscream said:
http://xcrforum.com/index.php/topic,3956.0.html
Click to expand...

Also an impressive post....:yingyang:
 
We (KAC) have made piston guns, and piston guns in the M16FOW, however note the PDW other than external controls, has no internal similarities to a M16FOW.

All guns foul, its the nature of the beast.

Semi-Auto and Auto systems also foul in the chamber due to extraction when the bore is not fully evacuated, its not usually due to the cyclic method, but due to the fact your extracting the case.
This is exacerbated in suppressed weapons due to the increased back pressure and the higher cyclic rate.
This is true for a DI or Piston gun.
I understand the method of operation is as tiriaq posted but for posterity sake I will call it DI as that is the accepted term.

As for the cleaning issue that incorrigable posted.
Well, the piston regardless of where it is fouls. It is is the bolt and carrier or the piston link up on an push rod.
Cleaning the M16FOW is more of a :bigHug::bigHug::bigHug::bigHug::bigHug: due to Army (Big Army and CF) beleives that it needs to be cleaned to the Nth degree, and the fact that the coatings on most M16FOW's are MaganesePhosphate, which is a good coating for wear resitance and corrosion resistance, however it is porous and loves to let carbon adhere.
There are coatings out there that clean up a lot quicker than that and work better. However for reliable operations, all you really need to do is add Lube.

I used to have a great afinity for the Hk416, and I am sure a lot of folks think my dislike is since I work for KAC. In some ways this is true, but its due to education, not any sort of company rivalry.
Pistons guns are much more sensitive to ammunition selections, and really finicky about adding cans. Add in the issues with carrier tilt, bolt lugs shearing (the M16FOW pressurizes the Bolt against the barrel extension when the gasses fill the expansion chamber. Piston guns simply smash a rod into the bolt carrier and all the load goes into the bolt lugs against the back of the barrel extension.
Hardly a bonus for bolt lug life.
Yes you can engineer tolerances and grind finishes, and use really exotic materials to try to defeat this, but you often find that trial weapons (and this is true to most companies I see) are cherry picked.
A gun that is alledged to have a 30k barrel and bolt life all of a sudden is having receiver crack and bolt lug issues under 3,500 rds (suppressed) in delivery guns - that is not acceptable.

The USMC IAR - is an area where Hk shines, it has a much larfger barrel nut to pull heat out of the barrel and chamber, and into the rail.
There is no suppressed requirment, and ammuntion for trial was all M855.
Now enter ATK's Mk317 62gr SOST round, and you may see some issues, especially if you add a can.

Don't get me wrong, I think Hk builds the best of the piston 5.56mm M16FOW systems, however I do not think it is the best of the M16FOW systems.

I have not even gotten into the recoil impulse issues with pistons and the Night Vision aids ;)

IF I was going to go piston, I would not due it in the conventional shaped M16FOW upper receiver.
It needs real bolt rails.


Fixing chamber area fouling, needs to be done by lengthening the cam - which cannot be done effectively in the current M16FOW bolt and carrier design, this is an area where the AK style cam path bolt carrier/bolt linkup is much eaier to monkey with.


This is as about as much as I can post on an open board.
 
Last edited:
There is some knowledge

KevinB said:
We (KAC) have made piston guns, and piston guns in the M16FOW, however note the PDW other than external controls, has no internal similarities to a M16FOW.

All guns foul, its the nature of the beast.

Semi-Auto and Auto systems also foul in the chamber due to extraction when the bore is not fully evacuated, its not usually due to the cyclic method, but due to the fact your extracting the case.
This is exacerbated in suppressed weapons due to the increased back pressure and the higher cyclic rate.
This is true for a DI or Piston gun.
I understand the method of operation is as tiriaq posted but for posterity sake I will call it DI as that is the accepted term.

As for the cleaning issue that incorrigable posted.
Well, the piston regardless of where it is fouls. It is is the bolt and carrier or the piston link up on an push rod.
Cleaning the M16FOW is more of a ##### due to Army (Big Army and CF) beleives that it needs to be cleaned to the Nth degree, and the fact that the coatings on most M16FOW's are MaganesePhosphate, which is a good coating for wear resitance and corrosion resistance, however it is porous and loves to let carbon adhere.
There are coatings out there that clean up a lot quicker than that and work better. However for reliable operations, all you really need to do is add Lube.

I used to have a great afinity for the Hk416, and I am sure a lot of folks think my dislike is since I work for KAC. In some ways this is true, but its due to education, not any sort of company rivalry.
Pistons guns are much more sensitive to ammunition selections, and really finicky about adding cans. Add in the issues with carrier tilt, bolt lugs shearing (the M16FOW pressurizes the Bolt against the barrel extension when the gasses fill the expansion chamber. Piston guns simply smash a rod into the bolt carrier and all the load goes into the bolt lugs against the back of the barrel extension.
Hardly a bonus for bolt lug life.
Yes you can engineer tolerances and grind finishes, and use really exotic materials to try to defeat this, but you often find that trial weapons (and this is true to most companies I see) are cherry picked.
A gun that is alledged to have a 30k barrel and bolt life all of a sudden is having receiver crack and bolt lug issues under 3,500 rds (suppressed) in delivery guns - that is not acceptable.

The USMC IAR - is an area where Hk shines, it has a much larfger barrel nut to pull heat out of the barrel and chamber, and into the rail.
There is no suppressed requirment, and ammuntion for trial was all M855.
Now enter ATK's Mk317 62gr SOST round, or Black-Hills 70gr Brown Tip, and you may see some issues, especially if you add a can.

Don't get me wrong, I think Hk builds the best of the piston 5.56mm M16FOW systems, however I do not think it is the best of the M16FOW systems.

I have not even gotten into the recoil impulse issues with pistons and the Night Vision aids ;)

IF I was going to go piston, I would not due it in the conventional shaped M16FOW upper receiver.
It needs real bolt rails.


Fixing chamber area fouling, needs to be done by lengthening the cam - which cannot be done effectively in the current M16FOW bolt and carrier design, this is an area where the AK style cam path bolt carrier/bolt linkup is much eaier to monkey with.


This is as about as much as I can post on an open board.
Click to expand...

Kevin,

Explained well.

Greg
 
Kevin knows what he is talking about but as always my two cents :) The advantage that I see in my experience of Piston vs DI AR15s (fun range shooting)

I go to the range and maybe shoot 100 to 250 rds at a time. In my DI gun the bolt/carrier is dirty with baked on carbon and all the lube is gone, its bone dry.

My piston AR`s which I have two,one long stroke PWS and one short stroke PWS. The bolts are still dirty BUT no baked on carbon and the grease is still on the bolt and carrier, its still wet and shiney with lube.

I think the worst flaw of the DI AR15 is the heat that gets transfered to the bolt/carrier and reciever resulting in hard carbon deposits and drying out the lube so they tell you to add more lube which makes more carbon and gets it even more dirty.

I like my PWS piston uppers they work perfectly and I see no wear at all anywhere, I probablly will get Questar to convert my last DI upper too. I like just running a couple patches with my Otis kit thru the barrel for cleanup and not feel like I have to clean/lube my bolt all the time like I used to in the DI uppers.
 
OZZ said:
Kevin knows what he is talking about but as always my two cents :) The advantage that I see in my experience of Piston vs DI AR15s (fun range shooting)

I go to the range and maybe shoot 100 to 250 rds at a time. In my DI gun the bolt/carrier is dirty with baked on carbon and all the lube is gone, its bone dry.

My piston AR`s which I have two,one long stroke PWS and one short stroke PWS. The bolts are still dirty BUT no baked on carbon and the grease is still on the bolt and carrier, its still wet and shiney with lube.

I think the worst flaw of the DI AR15 is the heat that gets transfered to the bolt/carrier and reciever resulting in hard carbon deposits and drying out the lube so they tell you to add more lube which makes more carbon and gets it even more dirty.

I like my PWS piston uppers they work perfectly and I see no wear at all anywhere, I probablly will get Questar to convert my last DI upper too. I like just running a couple patches with my Otis kit thru the barrel for cleanup and not feel like I have to clean/lube my bolt all the time like I used to in the DI uppers.
Click to expand...

If you run your bolt and carrier wet, 100-250 rounds is nothing and is a simple wipe down cleanup on an AR. If I only shoot 100-250, my bolt and carrier is still wet at the end of the day. How do you figure that adding more lube makes more carbon? If anything, the oil suspends the carbon instead of letting it get caked on our bolt and carrier. Maybe you're not running wet enough.
 
FPembleton said:
If you run your bolt and carrier wet, 100-250 rounds is nothing and is a simple wipe down cleanup on an AR. If I only shoot 100-250, my bolt and carrier is still wet at the end of the day. How do you figure that adding more lube makes more carbon? If anything, the oil suspends the carbon instead of letting it get caked on our bolt and carrier. Maybe you're not running wet enough.
Click to expand...




Its pretty simple more cooked oil/grease equals more buildup. I dont need/feel the need to clean or lube my piston bolts at all after a range use. Just the chamber and barrel with a Otis patch, takes 2 minutes and back in the safe . The DI AR treated the same way is bone dry when I take it out again.

So you go to the range and shoot what 1000rds everytime? The AR bolt is smoking hot after 10 10rd mags fired at a semi rapid pace.
 
If you only shoot at the range buy the cheaper AR's their great guns. If you want to shoot gophers and 'yotes with a .223 then swap to the 7.62x39 barrel and go hunt deer buy an XCR with a piston for a little more money. Simple.
 
FPembleton said:
If you run your bolt and carrier wet, 100-250 rounds is nothing and is a simple wipe down cleanup on an AR. If I only shoot 100-250, my bolt and carrier is still wet at the end of the day. How do you figure that adding more lube makes more carbon? If anything, the oil suspends the carbon instead of letting it get caked on our bolt and carrier. Maybe you're not running wet enough.
Click to expand...

If you run it wet you can easly put over 1000rds through it. "C8A3"
 
I thought I read that the MK18 only had a bolt life of 3 to 5 K. I agree that supressing a gun changes everything. I'm not convinced about the "stresses" on the bolt of a piston / vs / a DI system pressurizing the bolt against the barrel extension. If that was the case and Piston AR's were breaking bolts all the time then the HKG36 would break bolts all the time as well. G36's don't. The HK416 and G36 have the same gas system and bolt. I'm going to chalk up the piston AR bolt breaking to some of the non-mil spec cheap piston AR's.

In any case I'd like more info on the Sig Sauer USA 516 AR. I've been told that the lead designer of the HK 416 was hired by Sig USA. He designed the 516 and "corrected all of the problems he made on the HK416". There is also said to be an option of having a "supressor gas setting" on the select fire Sig 516.

Rich
 
Rich LPS said:
If that was the case and Piston AR's were breaking bolts all the time then the HKG36 would break bolts all the time as well. G36's don't. The HK416 and G36 have the same gas system and bolt. I'm going to chalk up the piston AR bolt breaking to some of the non-mil spec cheap piston AR's.

In any case I'd like more info on the Sig Sauer USA 516 AR. I've been told that the lead designer of the HK 416 was hired by Sig USA. He designed the 516 and "corrected all of the problems he made on the HK416". There is also said to be an option of having a "supressor gas setting" on the select fire Sig 516.

Rich
Click to expand...

The G36 is a completely different system. The operating principles may be similar however the G36 was designed from the ground up around an AR18 styled bolt and gas piston design. It's BCG on the recoil spring guide and IIRC there's also steel guide rails inside the body, so it hardly has to worry about the same issues as a piston AR.

And I'm not entirely sure how you're mixing up this:
HKBCG_01.jpg


with this;
g36boltgroup-2.jpg


There's no way the bolts can be the same. The lug that controls when the bolt unlocks are 90 degrees to each other...
 
Thanks for the tip and the pictures Starscream,

You still haven't addressed how the gas pressurization of the bolt in a DI gun would be any different when not pressurized and effecting the HK416 or G36 in the same fashion. The Gas systems, bolt lugs and locking up are exactly the same in the 416 and G36. I'm well aware that the rear portions of the bolts, bolt carriers and recoil mechanisms are different.

Not a lot of confusion. I don't need to down load internet pictures as I own both a G36P and HK416 and can look, shoot at and touch the systems in detail.

Rich
 
I feel that attaching the piston to the gas key and utillizing long stroke instead of short stroke is a simple way of fixing the breakage problems.However I don't see any advantage in a piston driven system anyway.
 
Rich LPS said:
Thanks for the tip and the pictures Starscream,

You still haven't addressed how the gas pressurization of the bolt in a DI gun would be any different when not pressurized and effecting the HK416 or G36 in the same fashion. The Gas systems, bolt lugs and locking up are exactly the same in the 416 and G36. I'm well aware that the rear portions of the bolts, bolt carriers and recoil mechanisms are different.

Not a lot of confusion. I don't need to down load internet pictures as I own both a G36P and HK416 and can look, shoot at and touch the systems in detail.

Rich
Click to expand...

If I understand the problem correctly it's an impulse problem. The DI allows a longer build up of momentum so that by the time the peak pressure is reached the bolt is already in motion, and then is bleeding off pressure before the bolt fully unlocks. It's pushed more gently. Plus as the carrier key disconnects from the tube the pressure rushes into the chamber and BCG that traps some of the gasses and helps keep the BCG aligned.

GP is a very high impulse over a shorter time, like a hammer slamming into the key. More impulse, more momentum, translates into a bolt that's got more force acting on it in that brief period. Net wise, it's the same amount of energy but it's acting over a shorter period of time and over a lot less area. Dedicated systems like the G36 mitigates the problem by having a cut that allows a few milliseconds of rearward travel before it unlocks. By the time the bolt lugs feel any force the short stroke of the gas rod has already ended and the bolt is riding on only the momentum of the BCG traveling.


http://img.photobucket.com/albums/v11/DavePAL84/projects/tilt1.jpg
http://i162.photobucket.com/albums/t244/seth_100/DSCN1866.jpg

Far as I know HK416 mitigated the problem by beefing up the rear of the carrier and tapering the end 30° inward. They also had to include a slightly modified buffer and buffer spring to mitigate buffer battering and wear.

For others, there's been efforts to reduce the tilt and battering:
http://i162.photobucket.com/albums/t244/seth_100/DSCN1845.jpg


I'm not saying that the HK416 is a bad system. It's well engineered, and HK does know their stuff. But they're trying to retrofit a gas piston into a chassis that was never designed for it, and that almost never ends well.

From a value standpoint, if you want a gas piston system, buy an SL8 with an AR magwell, a Swiss Arms, an XCR, an AR-180B; any of those systems will run with less stresses then a retrofitted AR because they were designed from the ground up around those systems. Only reason to buy a GP-AR is if it makes you happy, sorta like buying a Ferrari.;)
 
The 416 bolt carrier also has a cut that allows for a few miliseconds of travel before the bolt starts to unlock. The 416 does develop some visable wear spots that tend to induce shock in some shooters. I don't think that they are that serious. The Marines were very impressed with the HK416 and make no mistake in saying that it easily out performed the DI operated Colt and FN SCAR products by a wide margin (the SCAR should have been submitted with just a heavy barrel and not that crazy spring / heat / open bolt system thing).

Rich
 
Every open and honest trial done between piston and DI usually ends up with DI guns winning.

Brit SharpShooter
Uk SOF 5.56mm selection of the C8SFW and C8CQB

and some other recent programs ;)


Pistons can look very appealing at the outset, however they have a lot of flaws too, and in the last 50 years we learned a lot about DI guns, and how to improve them way better than a piston band-aid...



Coatings and Cam path designs can do a lot more than a sharp wrap from a piston..
 
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