GUN Some piston vs. DI thoughts

Discussion in 'On Topic' started by THT, Nov 28, 2009.

  1. THT

    THT The easy way is always mined

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    I took a couple of my ARs out on Thanksgiving and noticed a few things shooting my 14.5" long-stroke piston and my 16" midlength DI.

    *The piston system has a strangely lessened felt recoil than the middy. Both used heavy buffers and the same ACS stock with enhanced recoil pad as well as PWS556 compensators. I was expecting it to be snappier/harsher than the DI but was pleasantly surprised. My observation was independently confirmed by four other shooters.

    *Even with a midlength, I was still getting gas face intermittently. This was obviously not an issue with the piston upper.

    *The DI was the more accurate of the two at 100yds. It did have the benefit of a free float rail whereas the piston system by nature was not.
     
  2. Soybomb

    Soybomb New Member

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    conclusion:

    shooting stuff is fun? :big grin:
     
  3. fatmoocow

    fatmoocow bored OT Supporter

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    am I the only one who hasn't experienced any "gas face" issues with DI? Are you running suppressed?

    I think pistons are a good idea, but I'd most like to see a short bcg with the recoil spring integrated into the piston system to allow folding stocks ala an ACR.
     
  4. THT

    THT The easy way is always mined

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    :werd:
    Not suppressed. I've had gas face issues with carbines and middys, mostly with surplus ammo.

    I'm going to order an LMT piston upper to complete my other demo rifle build so I can compare the long stroke vs. the short stroke piston systems.
     
  5. dpixel8

    dpixel8 New Member

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    piston is way too expensive for my tastes. I'll wait quite a while for when they go down in price
     
  6. THT

    THT The easy way is always mined

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    Well you're cheap. And yeah, they are pricey.

    And you're short.
     
  7. dpixel8

    dpixel8 New Member

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    [​IMG]
     
  8. Ebtromba

    Ebtromba Active Member

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    wait...piston uppers cannot be free-floated?
     
  9. dpixel8

    dpixel8 New Member

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    sure you can
     
  10. THT

    THT The easy way is always mined

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    Not really. With a DI, there's the gas tube running back from the gas block while the piston has moving parts clamped to the barrel that are affected by gas pressure.
     
  11. JRucker2004

    JRucker2004 New Member

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    after running the piston for a while, You'll get to look forward to this:

    [​IMG]

    [​IMG]
     
  12. THT

    THT The easy way is always mined

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    I talked to Chris at Addax about that and he hasn't had that issue with the long-stroke system.
     
  13. JRucker2004

    JRucker2004 New Member

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    I hope you're right, but it's still got the problem of being pushed back from an off center position. The AR bolt wasn't designed to be used like that. The DI system pushes back from all the way around, essentially from the middle of the bolt, instead of the top.
     
  14. THT

    THT The easy way is always mined

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    There're a couple units in the field right now undergoing long-term torture testing and I know there's one with 12K+ on it with no maintenance other than a bore snake run through it every few thousand rounds but no issues or failures yet.
     
  15. Ebtromba

    Ebtromba Active Member

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    yeah seems like this would be a big deal the way everyone ball washes a free-floated handguard.
     
  16. Cannondale

    Cannondale OT Supporter

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    Stoner had a great idea and made the AR-15, I see no reason to change it.
     
  17. ChipOnShoulder

    ChipOnShoulder New Member

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    Surpressors probably weren't on his mind when he designed the original system. Neither were short barrels. James Sullivan, one of the engineers who worked with Stoner to downsize the AR-10 to the AR-15 noted that they designed the weapon and ammunition as a whole. The carbine gas system was an impromptu project by Colt after Stoner was no longer involved in the development of the weapon.

    Anyway, i'd seriously consider getting a piston upper just because of the PITA with suppressor use, but it'd probably be cheaper and smarter to just go with a Noveske switchblock (or similiar gas adjustment system) and not have to blow a lotta money, or spend money on a whole new upper.
     
  18. fatmoocow

    fatmoocow bored OT Supporter

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    not really
     
  19. Explain. The gas comes in the key, and acts on the bolt from all around. The op-rod in a piston system pushes only at the very top of the carrier, the furthest point possible from the BCG centerline.
     
  20. Keesh

    Keesh New Member

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    [​IMG]
    This looks like a pretty simple fix for carrier tilt. With all the different designs, I would say it's a non-issue and the pros vastly outweigh the cons if you're shooting suppressed.
     
  21. JRucker2004

    JRucker2004 New Member

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    If you're changing to piston because you're running a suppressor, I guess you need to weigh the pros vs cons. It may leave the upper a little cleaner, but it's also louder. You're venting gases from the gas port almost directly to the atmosphere, where in DI, the gasses are allowed to expand (through the gas tube, and inside the carrier) before being vented.
    If you're going for as quiet as possible, DI is going to have a slight advantage, but I can understand the benefits of being more reliable.

    As for the pic Keesh posted, it's treating the symptoms and not the problem. The carrier will push down on the buffer, which will then wear into the buffer tube. Seems like a poor solution to me.
     
  22. Keesh

    Keesh New Member

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    I think the main reasons for using a piston while suppressed is less wear on the system, no gas in the face, and increased reliability.

    The buffer I posted has been around for almost two years now and it seems that the guys using them haven't had any problems. I don't think there's much else you can do, short of redesigning the whole platform.
     
  23. JRucker2004

    JRucker2004 New Member

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    Which is exactly what I'm saying needs to be done (and has already been done: SCAR, Masada, XCR might fit in this list too)
    Adding parts to, and modifying an existing design can be an excellent way to improve upon it, but IMO, in this case there haven't been any good innovative solutions to the existing problems YET. So far all I've seen is taking ideas from other rifles, and slapping them on the AR. But gun guys are an innovative bunch, I expect there to be new ideas out there before too long that will be better than both DI and piston for all around use.
     
  24. fatmoocow

    fatmoocow bored OT Supporter

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    Gas pushes on key which is mounted to the top of the bolt. Force is still applied from roughly the same point in the same direction. The major difference is that the piston has direct contact with the "key" (or thing that sticks out like a key) through a larger portion of the motion.
     
  25. JRucker2004

    JRucker2004 New Member

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    Sorry, but that's not how DI works.

    The gas doesn't push on the key at all, the key is simply another tube that directs gas behind the bolt. Look at an ar15 bolt, specifically the back of it.
    Notice the smallest diameter part is shiny, that's because it seals with the bolt carrier. Same as the three rings right in front of that. Now, when the bolt is inserted into the carrier, it creates an expansion chamber. The gases are routed into this chamber, and as the gasses continue to expand, the bolt carrier is pushed back (equally, from all surfaces inside the expansion chamber) while the bolt stays where it is. As the carrier moves back, the cam pin rotates and unlocks the bolt. After the carrier moves back far enough, the vent holes (you can see on the side of the carrier) are exposed, and the gasses are vented out of the expansion chamber.
     

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