Explain the piston system to me...


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zstephens13
January 11, 2011, 11:41 PM
I have a full and complete understanding of the DI operating system for the AR type rifle.

So I get that the gas goes to the gas block and down the gas tube and PUSHES the bolt carrier down the reciever extention tube and the spring throws it back to the front that picks up and chambers a new round and you're ready to fire.

How does the piston system work? With the Rock River PDS there is obviously not a spring in the stock required to fire.
Basically, I don't get it.

Help?

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griff383
January 11, 2011, 11:47 PM
The piston rod essentially replaces the gas tube. Instead of the gasses traveling back to push the bolt the piston rod does this. The gas goes into a special gas block and pushes on the piston rod less than an inch from the gas port. This is why it is a cleaner and cooler (temperature) method of operation.

I dont have any knowledge on the RR PDS so I cannot answer that question, sorry

Jaws
January 11, 2011, 11:56 PM
Watch this animation. and than click on "compare to Ar15:

http://www.armytimes.com/projects/flash/2007_02_20_carbine/

In other words. The piston system keeps the hot dirty gases that cycle the weapon away from the action, keeping the internals cleaner and cooler.

cuervo
January 11, 2011, 11:57 PM
There is a spring some place, just not in the stock like on AR15s.

In this case, it is under the rail system.

Looking at their brochure

http://www.rockriverarms.com/images/products/rra_lr1222.pdf

the operating handle is over the hand guard. Pulling the handle back, or firing the rifle, compresses the spring to open the bolt and extending the spring closes it.

Once the piston is extended, the gas can be bled off someplace and not have to go into the chamber.

Tirod
January 12, 2011, 01:55 AM
Direct impingement drives the carrier back, yes. For every action there is an equal and opposite reaction. Since the carrier is the gas cylinder, what does it push against? The bolt tail with gas rings. That's a stationary gas piston, with traveling cylinder. That's how it dispensed with the operating rod.

Since the piston IS the action, it gets just as dirty. Thermal measuring shows the gas piston on the barrel runs much higher temps than the gas piston in the DI setup. It's closer to the source, and doesn't get gas directed down a tube cooling it.

In DI, the bolt carrier only runs about 30-45 degrees warmer, and the same thermal measuring tests show the bolt picks up most of it's heat because hot brass in pressed against the face.

The facts are: gas piston guns runs hotter gas pistons than DI does. Gas piston guns get dirty gas pistons, and gas piston guns are harder to clean because you have to disassemble the gas action on the barrel in addition to the bolt carrier.

How is it that disabling the DI gas piston in the bolt carrier just to add one that runs hotter and gets just as dirty, is somehow cooler and cleaner?

Ohhh, right, it's marketing, tell a lie often enough and people believe it regardless of the facts. Ironic that the same people who first practiced it invented the assault rifle.

No, gas piston guns are not cooler and cleaner. They get hot and dirty in a different place is all.

zstephens13
January 12, 2011, 06:54 PM
Tirod, thank you for all that information, but that didn't address any of my questions.

With DI, the bolt carrier group goes INTO the reciever extention tube.
With the RRA PDS there IS NO reciever extention tube.

So where does it go? Does the bolt carrier group look different? i.e. shorter or smaller.
How does it pick up a new round and chamber it?
It seems like there is no room for the operation to take place?

Can anyone speak to those?

Bartholomew Roberts
January 12, 2011, 07:40 PM
So where does it go?

Instead of a "push" spring at the back, there is a "pull" spring at the front over the barrel. You see the extended raised section of rail? The spring is underneath there, probably wrapped around the oprod.

Does the bolt carrier group look different? i.e. shorter or smaller.

Yes, it probably is shorter and there are probably other modifications to the bolt carrier.

Z71
January 12, 2011, 08:44 PM
The system I don't fully understand is the Cetme/HK style 'delayed blowback' system.

Works great..however it works.

Near as I can tell..it the chamber flutes reduce case friction by 'floating' the empty case...the heavy bolt carrier is more or less pushed backwards by recoil..but the rollers force the bolthead forward at the same time..and must account for the delay...just boggles the mind.

longdayjake
January 12, 2011, 09:23 PM
I imagine the RRA system is similar to the garand. It must have some form of oprod that is attatched to the bolt that yanks it back into place.

iamkris
January 12, 2011, 10:08 PM
The system I don't fully understand is the Cetme/HK style 'delayed blowback' system.

Works great..however it works.

Near as I can tell..it the chamber flutes reduce case friction by 'floating' the empty case...the heavy bolt carrier is more or less pushed backwards by recoil..but the rollers force the bolthead forward at the same time..and must account for the delay...just boggles the mind.

I used to have a series of drawings that illustrated it pretty well but can't locate them now. Here's the best I can find.

http://world.guns.ru/userfiles/images/assault/as60/cetme_pat.gif

Net-net, the bolt is 2-part [19 and 20] and contained in the carrier. At firing the bolt head [20] starts moving back and the rollers are moved outward (and semi-lock into recesses in the receiver) by the inclined surface of the other part of the bolt [19c] (I think they call it a cocking head). My then the larger carrier [12] is moving and moves back as the pressure at the cartridge diminishes. After a small movement the cocking piece moves back with the carrier, unlocking the rollers and the whole she-bang moves back and extracts / ejects the cartridge. The flutes in the barrel help unstick the cartridge from the chamber and have no part in the locking sequence.

Happy to be corrected if I got some part of this wrong.

Z71
January 12, 2011, 10:57 PM
The chamber flutes are the one thing I understand..the flutes allow pressurized gas to flow around the outside of the case... helping to offset the pressure inside the case... which is trying to keep the case sealed tight in the chamber. This being neccesary because the design requires some rearward case movement before the bullet leaves the barrel.

Without flutes to help equalize the pressure inside and outside the case..the case cannot start moving backwards with the bolt.

Not sure if that makes sense or not..but basicaly the flutes allow the fired case to move rearwards under pressure. That's the only part I really understood in gunsmith class
...the piston systems I understand.

451 Detonics
January 12, 2011, 11:42 PM
The piston system also suffers from carrier tilt, a problem no one has solved in a non-railed receiver. This is what one of the best in the industry, John Noveske said on the subject

"Also, we should mention the poor choice of platform for the piston conversion on a round receiver bore as found on the M16/M4 system. All other piston type systems out there utilize a railed receiver design, like the M14, AK-47, M249, FAL and so on. The round receiver bore design used on the M4 is only acceptable for the standard op system. The carrier and bolt expand on axis with the bore under the normal gas impingement cycle, but on a piston gun, you run into off center impulse issues with carrier tilt and incorrectly designed carrier contact points.

In other words if it ain't broke, don't fix it...

d2wing
January 12, 2011, 11:53 PM
I for one appreciate that information 451. Nice to have a solid reference and useful information.

Tirod
January 13, 2011, 12:48 AM
Cuervo answered it first, I saw no point in addressing the spring again. It's around the op rod forward. Others don't bother and use the standard action spring.

In whatever way it's done, the major point is to deactivate the bolt/piston and carrier cylinder arrangement of DI and add another on the barrel.

They are both gas piston actions, just like a Mauser or straight pull are both a bolt gun. It's the claim they are cooler and cleaner that's true hype, as the gas piston on the barrel kits are blown down with gas just like the Stoner system. All gas pistons get dirty.

It's just that some want to pay more and then not clean them. Well, don't clean the DI bolt carrier, and it will go thousands of rounds.

I just don't see why some think they are getting away with anything. I did own a HK91, and used the M16 for 22 years, so my experience is real world, not internet fantasy. They work. Nobody is offering a piston kit for the HK, I smell a lot of opportunism selling AR gas piston kits.

pitsmile
January 13, 2011, 01:57 AM
Watch this animation. and than click on "compare to Ar15:

http://www.armytimes.com/projects/flash/2007_02_20_carbine/

In other words. The piston system keeps the hot dirty gases that cycle the weapon away from the action, keeping the internals cleaner and cooler.
Good info. To answer the OP question, watch the video ^. That's the best diagram/vid I've seen.

The Rock River is probably moded so the spring is in the receiver, just my guess.

**Here is your answer... http://www.gunblog.com/rock-river-arms-pds-pistol/
...features an adjustable gas piston and an over barrel spring and rod arrangement.

blackops
January 13, 2011, 02:17 AM
In other words if it ain't broke, don't fix it...

This type of mind frame will surely leave us behind in the pack. Just because it isn't broke, does not mean it can't be improved. There will always be more technology, we must strive forward with it, or suffer the consequences. Look at Chinas new J-20. Our pilots are not thrilled to discover our F-22's have been surpassed.

Quentin
January 13, 2011, 03:37 AM
Like the man said, the M16/M4 ain't broke so don't fix it. When something really superior comes along then let's talk ... but recent "solutions" are tiny modifications that don't warrant dumping what you have and what works for something marginally different. Different, not superior and more likely inferior.

451 Detonics
January 13, 2011, 08:33 AM
This type of mind frame will surely leave us behind in the pack. Just because it isn't broke, does not mean it can't be improved. There will always be more technology, we must strive forward with it, or suffer the consequences. Look at Chinas new J-20. Our pilots are not thrilled to discover our F-22's have been surpassed.

Why change something that works very well into something that has problems. And then call it an improvement? Instead why not design a railed AR platform for use with a piston? To me that would be a step forward rather than being a step sideways and back. As I said the piston is an answer to a non existent problem. I have heard a shorty won't work well with DI...oddly enough my works perfectly. I heard they run better with a suppressor, my gun runs great suppressed and doesn't get any dirtier.

Most AR accessories are like fishing lures, they catch more fishermen than fish.

Jaws
January 13, 2011, 02:43 PM
The piston system also suffers from carrier tilt, a problem no one has solved in a non-railed receiver.

AR15 was not designed with a piston in mind. Most quick fix piston kits will be more headache than fix. Unless the company does some intensive redesign like in the case of HK416, you'll get more problems than you to fix.
Rifles desined with a gas piston from get go have rails for the bolt carrier to ride in. You won't have carrier tilt there.

RC20
January 13, 2011, 05:31 PM
Look at Chinas new J-20. Our pilots are not thrilled to discover our F-22's have been surpassed.

Really really bad bad uninformed comparison.

The J20 is a non fighting prototype. Any in service version is at least 10 years away. Anything effective (vs just flying) is 20 years out.

It also is not even remotely close to an F22 in radar signature or engines (let alone the network capability and radar.

It may not even be a fighter, as its F111 size it actually looks to be an attack aircraft. Given the need to penetrate sophisticated radar system and task forces that makes sense.

We are currently being robbed blind, probed and attack profiles being setup by their cyber warfare operations, and people get wigged out by photos released by their intelligence service. Hmmmm

RC20
January 13, 2011, 05:33 PM
AR15 was not designed with a piston in mind. Most quick fix piston kits will be more headache than fix. Unless the company does some intensive redesign like in the case of HK416, you'll get more problems than you to fix.
Rifles desined with a gas piston from get go have rails for the bolt carrier to ride in. You won't have carrier tilt there.

Yep, if you want a real piston, get an XCR! (well and a few other so so offering like an AK, SCAR or ACR)

Otherwise, for the non combat environment, if you don't mind the AR controls, the AR DI is fine (and some nice accurate guns I may add)

451 Detonics
January 13, 2011, 05:45 PM
AR15 was not designed with a piston in mind. Most quick fix piston kits will be more headache than fix. Unless the company does some intensive redesign like in the case of HK416, you'll get more problems than you to fix.
Rifles desined with a gas piston from get go have rails for the bolt carrier to ride in. You won't have carrier tilt there.

This was what Noveske had to say about the 416...

In fact, there’s now some evidence to the contrary, since the much vaunted and supposedly superior HK416 has been running into some rather serious reliability and durability problems in the field as of late, including the piston system locking up in cold weather and the upper receiver cracking. And, the HK 416 is supposed to be the most combat-proven gas piston/op-rod AR out there! According to some unconfirmed/unverified reports Defense Review has received from some of ourindustry contacts, the HK-416 receiver-cracking issue has led to U.S. Special Operations personnel operating under SOCOM (USSOCOM) to switch out their HK416s with Colt M4A1 Carbines, so they’re now back to using direct-gas-impingement guns. The fact is, the Colt M4A1 Carbine is truly combat proven, and significantly more so than ANY gas piston/op-rod AR out there, including the HK416, if you’re talking the total number of rounds fired through a given system/platform in actual combat.

Jaws
January 13, 2011, 05:49 PM
I agree that J20 is not exactly in F22 class but it still is quite a treat.
Remember, the Navy won't be getting F22's. They'll get F35's. The F35 has the wing loading and power loading of the Vietnam era strike plane F-105 "Lead Sled".
I doubt F35 will be able to protect the Carriers from something like J20.

Jaws
January 13, 2011, 05:57 PM
This was what Noveske had to say about the 416...


Well, his business is built around DI AR rifles. I wouldn't expect him to say anything else. ;)
When we see as many of his rifles in military around the world as HK416's, then we could find out if his rifles are really better than 416. :)

Darthbauer
January 13, 2011, 06:10 PM
More problems??? I guess not many of you guys have any LWRC's then.

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