Piston vs. Gas

legion · 09-17-2009, 11:49 AM

Yep, another AR-15 thread.

I'm thinking of converting my M4 over to be piston operated.

Is this mod worthwhile?

Which kit should I use?

scottmandue · 09-17-2009, 11:52 AM

Pfft... electric is the way to go!

RPKESQ · 09-17-2009, 12:40 PM

Let's start with basics. There is only gas. Both systems use gas as the generator of the motive force to cycle the weapon. They are different as to how the use the gas only by slight degrees.

In a (Stoner) AR-15 system as designed the gas flows from the gas aperture to a tube that leads back to the piston which is integral with the bolt. Gas pushes the piston, piston transfers that energy to the bolt which moves back and utilizing cam surfaces rotates the locking portion of the bolt which unlocks and continues back into full recoil (which then the recoil spring after being compressed, releases its stored energy and forces the bolt back into battery (locked)).

Positive attributes of this system is lighter weight and less machining of some parts or reduction of parts count (which are cheaper to produce).

Negative attributes are greater chamber/bolt area fouling, sensitivity to powder type and cleaning and the requirements for a recoil spring and housing protrusion in the rear of the action.

In an M-1, M-14, FN FAL, AK , Mini-14 type system the gas is bled off into a gas cylinder that is attached to the barrel. A "piston" which may be a separate part or just a machined end of the operating rod (or piston rod) is pushed back by the gas, which causes the operating rod to transfer the backward motion to the bolt which in turn moves backward on some cam surfaces which cause the bolt head to rotate and unlock the bolt, causing it to continue backwards until the recoil spring is compressed which then drives the bolt forward into battery.

Positive attributes of this system are reduced chamber fouling, low sensitivity to powder types and cleanliness, no recoil spring requirement for a behind the action recoil spring and housing, which enables a folding (or no) stock. Also large robust parts for durability.

Negative attributes are higher manufacturing cost and heavier weight.

If you can use the proper powder type and keep the weapon clean the Stoner system is reliable and lighter.

If a more expensive, heavier weapon with greater parts durability and greater reliability in poor conditions is required then the M-1, M-14, AK, system is desirable.

Military accuracy is totally independent from gas system type with proper construction. In a civilian world of competition, it is easier to build an accurate system with the Stoner gas system.

HardDrive · 01:06 PM

Interesting.

MotoSook · 02:41 PM

I think he's talking about the AR-15 conversion kits that have become popular, not simply the DI vs piston design.

IMO, the AR-15/M16 design was never meant to be a piston operated weapon, and so there are drawbacks to installing a kit. The biggest one is the carrier tilt which can lead to buffer tube wear. The piston rod/shaft impacts the carrier off center (non-coaxial load) causing the front of the carrier to tilt up and the rear to tilt down. Although there is an anti-tilt carrier design, it cannot eliminate the force that causes the tilting motion in the first place. So the force is still present. The anit-tilt carrier may minimize the buffer tube wear, but it will eventually wear. The tilting carrier can also wear on the upper receiver prematurely.

If you really hate to clean your rifle and don't mind changing out the buffer tube now and then, it's probably not a big deal. Personally, I like cleaning my gear and don't want to add extra parts to the rifle that can fail in the heat of use. It's a small probability, but I don't know if I think the advantages of a piston kit is enough when a DI rifle will work for thousands of rounds without cleaning, just oil periodically.

The two kits that seem to be big right now are the CMMG kit and the Adams Arms kit. The Adams Arms kit started out close to $500 and uses your carrier while they provide a key that you have to install. They are currently having a sale for the kit at about $250.

Adam Arms Kit Discussion:

http://www.ar15.com/forums/topic.html?b=3&f=124&t=374524

The CMMG kit comes with an anti-tilt carrier and key machined as a single piece. The latest version has a steel gas/piston block that appears to be a better design. I'm not certain it has been installed on a rifle length gas system, but CMMG can verify. It retails for $299, but AIM has it discounted $10 or so.

CMMG Kit Discussed:

http://www.ar15.com/forums/topic.html?b=3&f=124&t=452885

There are fewer choices of rails or hand guards due to the space requirement of the piston kit, but if you like the basic M4-esque hand guard the CMMG comes with it. S&W uses the Adams Arms kit on their MP15 rifles. Spike also uses the Adams Arms. CMMG also sells a complete piston upper for about $675 with a 16" 1/9 barrel. So you could have the best of both worlds if you keep the DI upper.

I may one day play around with a piston kit (the CMMG looks good to me), but for now I don't think it's necessary.

MotoSook · 09-17-2009, 02:25 PM

This is a basic but cool animation of DI vs Piston....

http://dl.getdropbox.com/u/1702248/mod1.html

HardDrive · 09-17-2009, 02:44 PM

Not to put too fine a point on it, but if the piston design is so superior, why isn't someone simply making them from the factory that way? Since you are using a standard AR-15 receiver, you could still call it an AR-15......maybe the AK-15? :P

I don't understand why the piston would need to contact the carrier at an angle. Could you just build the front mount slightly higher and make it align perfectly?

BlueSkyJaunte · 09-17-2009, 02:49 PM

I have heard good things about the PWS conversion, and the one I played with (their "Diablo" upper) ran like clockwork. However I don't know enough about how their system compares to the other conversion kits to form a real opinion.

I will say that my POF upper runs like a scalded dog and has never given me a hint of trouble, even with really dirty ammo. I run it dry and have done no maintenance on it other than to run a boresnake down the barrel every 500 rounds or so. I have heard bad things about POF from time to time but have never experienced them!

What is the goal of your conversion? Less cleaning? Greater reliability? I have 4 other ARs besides the POF and feel no great need to convert them to piston operation.

The #1 item on my list for all of them (except the .458 SOCOM....sigh) is the PWS FSC flash suppressor/comp. The first time shooting a rifle with that thing on it was downright WEIRD. The muzzle climb/recoil is improved so drastically that I thought the gun was broken.

MotoSook · 07:48 AM

Quote:

Originally Posted by HardDrive

Not to put too fine a point on it, but if the piston design is so superior, why isn't someone simply making them from the factory that way? Since you are using a standard AR-15 receiver, you could still call it an AR-15......maybe the AK-15? :P

I don't understand why the piston would need to contact the carrier at an angle. Could you just build the front mount slightly higher and make it align perfectly?

I don't think it'd be possible to use a standard AR15 upper receiver and carrier without changing how the force is transfered to the carrier. If you change the carrier, you then have to change the receiver.

Everything works in a nice coaxial arrangement in the DI AR, so unless a design comes along that imparts equal force on opposite sides of the carrier without degrading the equality of the force over time, there'll always be some drawback. The system/kit might then cost too much since it'll require more changes to the receiver and carrier.

If I had to design a new action system based on the design of an AR and a piston, I would make the carrier ride on rails that run the length of the upper receiver. Since there is little gas that makes it into the receiver in a piston design to foul things up, a nice tight rail at the 9 and 3 o'clock position with some high temp grease would run nice and smooth and prevent tilting in the buffer tube. Maybe steel rails attached to the sides of the standard forged aluminum receiver and simply machine two slots in the carrier to mate to the rail.

The ejection port and forward assist would be in the way at 3 o'clock, but I'd get rid of the forward assist and make room for the rail on that side up to the ejection port, integrating a deflector.

Edit: if you are familiar with the smooth side 9mm upper receiver, you can machine a slot on the receiver at the 9 and 3 o'clock positions along the length of the receiver. A steel insert (with rails) can them be screwed onto the receiver with the rails protruding into the receiver's carrier bore. A carrier without the notches for the forward assist can then be machined with slots at the 9 and 3 o'clock to ride on the steel rail inserts.

BlueSkyJaunte · 09-17-2009, 09:03 PM

Who's the limp-wristed girly-man who gave this thread one star???

emcon5 · emcon5's Garage

Still kind of unclear on the point. Yeah, you can have a AR without a stock, and the action stays cleaner, but how big of a deal is this?

I am horrible about cleaning guns, and I have shot many hundreds of rounds between cleanings and never had a problem, and frankly I have never noticed them getting all that dirty.

I also like the fact that the Stoner system really has no moving parts.

RPKESQ · 09-18-2009, 07:17 AM

Quote:

Originally Posted by emcon5

Still kind of unclear on the point. Yeah, you can have a AR without a stock, and the action stays cleaner, but how big of a deal is this?

I am horrible about cleaning guns, and I have shot many hundreds of rounds between cleanings and never had a problem, and frankly I have never noticed them getting all that dirty.

I also like the fact that the Stoner system really has no moving parts.

Not true.
The Stoner system has almost as many moving parts as a piston design. The Stoner system only reduces the parts count by 1 piece in most designs, 2 in some rare designs.

MotoSook · 07:49 AM

The conversion kits do not eliminate the buffer tube, spring, buffer and stock. This is a hotly debated topic, DI vs Piston. Some guys report buffer tube wear while others don't. I say it's a matter of time before the buffer tube requires replacement. It's an easy replacement, but still a drawback of the pistion kit on an AR.

Maybe simply supplying a steel buffer tube to be used with an anti-tilt carrier would be a simple fix. It would add weight but them so will the piston kit. At some point the rifle is in the heavy catagory and adding a few more ounces is not going to be noticed.

Quote:

Still kind of unclear on the point. Yeah, you can have a AR without a stock, and the action stays cleaner, but how big of a deal is this?

I am horrible about cleaning guns, and I have shot many hundreds of rounds between cleanings and never had a problem, and frankly I have never noticed them getting all that dirty.

I also like the fact that the Stoner system really has no moving parts.

MotoSook · 09-18-2009, 07:53 AM

Quote:

Originally Posted by RPKESQ

Not true.
The Stoner system has almost as many moving parts as a piston design. The Stoner system only reduces the parts count by 1 piece in most designs, 2 in some rare designs.

In the case of the piston kit on an AR there is the addition of parts. The piston and shaft are moving. The bushings of the Adams Arms kit for example has to be pressed into the upper receiver. I suspect it will wear over time. With all the kits you have springs that will also need replacing. The piston and cylinder will also wear. It would probably a long time to the point where gas leakage prevents proper cycling.

RPKESQ · 09-18-2009, 08:00 AM

Conversions are in most cases dependent on two factors: engineering and quality.

Most AR-15 parts makers are not equipped with an engineering staff and lab like the manufacturers themselves. Quality of parts is another issue. How many parts makers can test to destruction repeatedly and use the data to re-engineer successfully (see point #1).

I am not a fan of trying to convert an AR-15 Stoner system into a piston design. The receiver was never designed to be the rear guide for the operating rod. So some kind of bearing surfaces must be added. Nylon and plastic in most of the kits I have examined. Not great engineering there! Better to buy a piston design if that is what you want. The engineering is correct out of the box that way.

There are several choices for a great AR-15 piston design, Ruger SR-556, Sig 556 are two of the best.

RPKESQ · 09-18-2009, 08:05 AM

Quote:

Originally Posted by Soukus

In the case of the piston kit on an AR there is the addition of parts. The piston and shaft are moving. The bushings of the Adams Arms kit for example has to be pressed into the upper receiver. I suspect it will wear over time. With all the kits you have springs that will also need replacing. The piston and cylinder will also wear. It would probably a long time to the point where gas leakage prevents proper cycling.

I was speaking from a design standpoint. In which case the piston and shaft are one piece and no addition parts are needed. In a conversion you are making up design deficiencies and need more parts to make it work.

A properly designed piston system is mostly self cleaning and durable for several hundred thousand rounds before wear becomes a factor in reliability.

MotoSook · 09-18-2009, 08:12 AM

Quote:

Originally Posted by RPKESQ

I was speaking from a design standpoint. In which case the piston and shaft are one piece and no addition parts are needed. In a conversion you are making up design deficiencies and need more parts to make it work.

A properly designed piston system is mostly self cleaning and durable for several hundred thousand rounds before wear becomes a factor in reliability.

I know you were speaking of the piston design, and not a conversion kit. The original poster was asking about a kit to convert a direct impingment designed AR. I've been trying to steer the thread back to the original poster's question

MotoSook · 09-18-2009, 08:16 AM

Quote:

Originally Posted by RPKESQ

Conversions are in most cases dependent on two factors: engineering and quality.

Most AR-15 parts makers are not equipped with an engineering staff and lab like the manufacturers themselves. Quality of parts is another issue. How many parts makers can test to destruction repeatedly and use the data to re-engineer successfully (see point #1).

I am not a fan of trying to convert an AR-15 Stoner system into a piston design. The receiver was never designed to be the rear guide for the operating rod. So some kind of bearing surfaces must be added. Nylon and plastic in most of the kits I have examined. Not great engineering there! Better to buy a piston design if that is what you want. The engineering is correct out of the box that way.

There are several choices for a great AR-15 piston design, Ruger SR-556, Sig 556 are two of the best.

The Ruger SR-556 is just a factory designed and installed piston kit. It still uses the Stoner designed bolt and action with a modified carrier like that of the CMMG kit. So it's really not a new design based on a piston system like the SIG 556.

RPKESQ · 09-18-2009, 08:56 AM

Quote:

Originally Posted by Soukus

The Ruger SR-556 is just a factory designed and installed piston kit. It still uses the Stoner designed bolt and action with a modified carrier like that of the CMMG kit. So it's really not a new design based on a piston system like the SIG 556.

You are correct, Ruger has added this capability to a standard AR-15 design. But I am willing to bet the reliability and quality is head and shoulders above a simple kit conversion.

Looking at a Ruger receiver no plastic or nylon bearing surfaces are visable. Ruger does use a seperate piston and operating rod designe along with a easily cleaned and regulated gas cylinder design (all of which does increase the parts count over a stock Stoner system). Not too bad overall.

But for my money, I would agree that the Sig 556 is the better made and designed weapon.