The JAC bv system and how it works

Discussion in 'Classic Guns' started by G36fanatic..., Oct 30, 2012.

  1. G36fanatic...

    G36fanatic... Supporting Member Supporting Member

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    Boulder
    *please sticky*

    The JAC BV system


    This is for everyone who has ever wondered why there are still people that use these 20+ year old guns. The BV system, or Bullet Valve, can be found in many systems, but JAC and Asahi seem to be the most popular and simplest., and for good reason. In the JAC system, there is only a small number of moving parts. There is very little metal on metal contact, and the system runs on a regulated external power source*. The most common external power source is a Palmers low pressure regulator, and either a CO2 tank or a HPA tank. Both tanks can be found, and filled at most paintball shops.


    *the JAC system does have an internal tank to run off of, but it is not recommended.
    [​IMG]

    First off, here is an exploded diagram of how JAC internals break down.
    [​IMG]

    An external air source enters in through the buffer area.
    [​IMG]
    Back here there is also a fill valve just like modern GBBs if you want to run off of the internal tank. It is not recommended to use this, as it doesn't last near long enough for a game, and there is cool down.
    [​IMG]

    Here we have the basic internals of the JAC system.
    [​IMG]
    [​IMG]
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    [​IMG]

    Air comes in from the buffer tube, into the valve area, and when you pull up on the trigger, it lets air pass into the mainchamber. Meanwhile, a BB has been fed up into the subchamber, and is resting right behind the mainchamber/barrel O-ring.
    [​IMG]
    The above picture shows where the subchamber sits so that it can feed BBs through.
     
  2. G36fanatic...

    G36fanatic... Supporting Member Supporting Member

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    Boulder
    [​IMG]
    Here is where the O-ring sits. It just rests inside the mainchamber, and sits up against the barrel.

    [​IMG]
    Because the system is sealed, the mags have to be airtight to allow pressure to build up.
    [​IMG]

    now that there is a BB resting behind the O-ring, and the mag is sealed, the barrel starts to move forward because of the building pressure.
    [​IMG]
    [​IMG]

    While the opposing force of the recoil spring stops the barrel from moving forward, the pressure builds until the BB slips through the O-ring at high velocity. The recoil spring returns the barrel, and subchamber back to its original position, where the process starts all over again.

    Now how does the trigger work?
    [​IMG]
    [​IMG]


    The trigger works as a lever, pushing up on the Bullet Valve, and letting air into the system. In Full auto mode, the valve is simply held open. During semi auto is where the trigger tabs come into play. As the barrel moves forward, so does the metal piece that the trigger tabs rest on. The tabs are connected to a rod like piece that pushes the trigger off of the valve upon being returned to a neutral position.

    That is it for the stock JAC system. There are upgrades such as SCS barrels and LRBs. An SCS is a Spin Control System. They were produced by companies like KM, or they were custom made. They use a very high quality rubber bucking to put a backspin on the BB. Usually, some disassembly is required to tune them, but some people have drilled a small hole in the top of the receiver so it can be adjusted on the fly.

    Both systems require the use of an anti rotation collar to keep the hop up aligned properly. These usually consist of a wider section of outer barrel that has a slot cut into it with a set screw in the barrel to keep the barrel from spinning.

    The other hop up fix would be an LRB, or Long Range Barrel. These barrels are designed so that the BB rolls along the top of the barrel, creating a very consistent and powerful backspin. They usually require the use of .28g or .30g BBs. Some use heavier or lighter BBs depending on how aggressive the barrel is made.

    For more information about classic airsoft, please visit www.classicairsoft.org
     

  3. L5132

    L5132 New Member

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    Duluth
    Interesting, I hadn't taken much time to figure out how classics work.
     
  4. Thestig

    Thestig Some say... Supporting Member

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    Colorado Springs and Southern California
    Glad to see you back G36. This is a great read, and I learned a lot. Thank you!
     
  5. memorydmr

    memorydmr New Member

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    Awesome read!
    I learned that the barrel is the part that moves to give the recoil! Wow! Interesting design.

    I'd love to get one of these JAC BV guns to tinker with and apply some
    Modern tech to it to see about improving range and accuracy to them:)

    It's a shame these guns have gotten so buried over the years. It's a simple design that works.
     
  6. G36fanatic...

    G36fanatic... Supporting Member Supporting Member

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    Boulder
    Although the barrel moves, there really isn't any recoil. Some people add weights to the barrel, but that really messes with the system and you have to know what you are doing. Even when weights are added, the recoil doesn't increase much, and they generally aren't added for the recoil.


    One day I'd like to start a company that makes this system new again. The constant pressure behind the BB really gives these guns potential.
     
  7. memorydmr

    memorydmr New Member

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    True true, im guessing any recoil it does give isn't much more than the modern GBB rifle?
    It's more of a vibration I'm guessing than recoil.:)

    And that sounds like something that could give all the airsofters that like the BV system a chance to use it without breaking the bank!
    And just give more variety Airsoft systems to choose from!

    I'd love to see that happen! Best of wishes on that!
     
  8. G36fanatic...

    G36fanatic... Supporting Member Supporting Member

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    Boulder
    GBBRs have a recoil you can feel. Classics with a BV engine do not have any sort of felt recoil. The BV engine is so simple and cheap to produce, but he cost of an external air tank and regulator would nullify the cost.