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Explain the piston system to me...

Discussion in 'Rifle Country' started by zstephens13, Jan 11, 2011.

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  1. zstephens13

    zstephens13 Member

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    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?
     
  2. griff383

    griff383 Member

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    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
     
  3. Jaws

    Jaws Member

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  4. cuervo

    cuervo Member

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    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.
     
  5. Tirod

    Tirod Member

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    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.
     
  6. zstephens13

    zstephens13 Member

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    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?
     
  7. Bartholomew Roberts

    Bartholomew Roberts Moderator Emeritus

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    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.

    Yes, it probably is shorter and there are probably other modifications to the bolt carrier.
     
  8. Z71

    Z71 Member

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    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.
     
  9. longdayjake

    longdayjake Member

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    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.
     
  10. iamkris

    iamkris Member

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    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.

    [​IMG]

    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.
     
  11. Z71

    Z71 Member

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    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.
     
  12. 451 Detonics

    451 Detonics Member

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    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

    In other words if it ain't broke, don't fix it...
     
  13. d2wing

    d2wing Member

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    I for one appreciate that information 451. Nice to have a solid reference and useful information.
     
  14. Tirod

    Tirod Member

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    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.
     
  15. pitsmile

    pitsmile Member

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    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/
     
    Last edited: Jan 13, 2011
  16. blackops

    blackops Member

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    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.
     
  17. Quentin

    Quentin Member

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    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.
     
  18. 451 Detonics

    451 Detonics Member

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    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.
     
  19. Jaws

    Jaws Member

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    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.
     
  20. RC20

    RC20 Member

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    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
     
  21. RC20

    RC20 Member

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    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)
     
  22. 451 Detonics

    451 Detonics Member

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

     
  23. Jaws

    Jaws Member

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    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.
     
  24. Jaws

    Jaws Member

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    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. :)
     
  25. Darthbauer

    Darthbauer Member

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    More problems??? I guess not many of you guys have any LWRC's then.
     
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