Joule creep question.

Discussion in 'Gun Building, Modifications & Repairs' started by Splode, Dec 22, 2018.

  1. Splode

    Splode Well-Known Member

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    Need a joule wizard to explain something to me. I understand what joule creep is, but I am wondering how a gun can use heavier ammo and NOT get any joule creep.:confused:
    In my BAR 10 sniper rifle with hop on, I shoot 2.66 joules with 20s (535 fps). I shoot 2.68 joules with .40s (380 fps)
    Thats practically nothing. Most the guys at the local fields have an advantage over me with moderate amounts of joule creep. Why is mine not benefitting at all? I was under the impression EVERY gun gets joule creep when using heavier ammo because the bb stays in the barrel longer.
     
  2. Splode

    Splode Well-Known Member

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    @1tonne
    Youve done a lot of work with bar 10s if i remember right. Is it possible im undervolumed? Im using the stock cylinder and head but everything else is Action Army including the barrel which I think is around 420mm. Im assuming since im getting like 560 fps (hop up off) with only a m150, that my compression is fine. I understand that the heavier bbs could be causing a leak to show itself but i dont know. The cylinder head is tephlon taped and so is the bucking. The action army hop up usually is tight enough around the bucking that you dont have to though.
    I saw in an old guide of yours that you shrink tubed the nozzle. Maybe that could help.
     

  3. 1tonne

    1tonne Well-Known Member

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    The barrel length is most likely fine. What bb weight are you using?

    Air leaks do effect JC a lot. Put your finger over the inner barrel with no bb in and fire. The air should stay in the rifle for at least 3 seconds. That way you know you have a pretty good air seal. My rifle does not leak at all.
    What I think is your issue is the spring is too small and you most likely need a heavier piston. You are getting good efficiency out of your spring but it is not always so great for joule creep. If you put in a bigger spring and wrap some solder or golfers tape around the piston shaft to weigh it down, you should then see some JC. What this will do it make the piston travel slow in its initial movement (this is when the 0.2gm should travel through and exit the inner barrel) and then the piston accelerates to maximum speed as it travels through the cylinder (this is when the heavy bb should exit the inner barrel. At maximum velocity)
     
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  4. Nilye

    Nilye New Member

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    1tonne please correct me if I am wrong -
    I am not very seasoned airsofter but from my own experience I can tell that JC will show when using short barrel and relatively heavy cylinder. Perfect air seal will help a lot but this point you already have.

    Heavy cylinder with its big inertia will push light bbs earlier and heavy bbs will stay a bit longer.
    Short barrel (about half you have now) will help because air pressure will affect light bbs shorter time than hevy ones.
    Michal
     
  5. 1tonne

    1tonne Well-Known Member

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    A heavy piston is good but it can sometimes be too heavy. If it is too heavy, for the spring then the piston may accelerate a little at the start and then only accelerate a little bit as it travels though the cylinder.
    Also, you are best to have a perfectly matched inner barrel length for the heavy bb weight. This will give you the most efficient joule creep build. You can use an even shorter barrel and you will get more of the joule creep effect but then you will start to lose a lot of energy as the heavy bb will be exiting the inner barrel before the piston is at maximum speed. The good thing about this is that the 0.2gm bb also exits early too and it does not get up much speed. But overall, JC should increase if done with the correct spring and piston weight.
     
  6. Splode

    Splode Well-Known Member

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    im using 40s but am thinking of trying 43s. The piston is blue. Action army i think. M150 spring. I didnt really want to use anything over that because im already at 520 fps which is almost field limits (530). Doesnt a heavier piston increase fps though?
    Ill do the finger test tomorrow and see.
     
  7. Splode

    Splode Well-Known Member

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    But i should still be getting SOME joule creep right? With 40g bbs and a 420mm 6.01 barrel and m150 spring? If i should at least be getting something then i must have a leak that only shows itself at high pressure when using HW bbs.
     
  8. 1tonne

    1tonne Well-Known Member

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    No. All the weight does is slow the piston down. It will slow the piston down more in it's initial movement and in the later movement too but not as much. Overall, the will bring the fps down

    Either you have a leak or the M150 spring just is not powerful enough. (Or both)
    For fun, try wrapping some solder around the piston shaft and hold it on with one layer of electrical tape. Then see the difference.
     
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  9. Splode

    Splode Well-Known Member

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    If the spring wasn’t powerful enough, then how could a m150 be shooting at 550ish with 20s? I think the first time I chronoed, the gun was overhopping 40s. I’ve backed it off a bit now.
     
  10. 1tonne

    1tonne Well-Known Member

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    Because you piston is light. The fact that you can get 550ish most likely means you have a good air seal too.
    Good air seal + light piston = good fps​
     
  11. Nilye

    Nilye New Member

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    Today I ve made an experiment.

    I have guarder sp140, Maxx TE hopup chamber, ML Hotshot 50dgree and 43g alu piston set. I swapped only 180mm and 410mm MLCJ barrels and chronoed with .2 and .45 bbs.
    Here is what I got:

    180 mm .2-360fps .45-275fps -->.38J JC
    410 mm .2-475fps .45-315fps -->.03J JC

    As I understood you 1tonne - JC is dependent of barrel length too. You elaborated this subject clearly on one of the topic in ASF. This was the reason I bought those barrel lengths - I felt you are not mistaken. 180mm for JC and 410mm for efficiency.
    And my experiment proved you are spot on this matter.
    Thank you
    Michal
     
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  12. 1tonne

    1tonne Well-Known Member

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    For those that do not know, Nilye is talking about an AEG with a full cylinder.
     
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  13. Lamducbao

    Lamducbao New Member

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    Hi, i was wondering if i want to use a short barrel in a aeg, and i dont want to lose fps with heavy bb due to short barrel, can i just increase the piston weight to get back the lost fps?
    Currently im shooting 295fps 0.32g m130, 274fps 0.32g m120, both are 200mm( stock barrel, maybe 6.05~6.08mm)
     
  14. 1tonne

    1tonne Well-Known Member

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    That can help but it depends on a lot of things. You will also need to use a bigger spring if you weight the piston down.
    The idea of JC is to make the piston travel slow at the start of it's movement (when the 0.2gm bb's will start to move) and then accelerate. You want the heavy bb to exit just as the piston slams into the cylinder head. So the light bb using the energy from the piston when the piston is traveling slow and the heavier bb will use the energy from the piston when the piston is travelling faster.
     
  15. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    ^This is the Law of Inertia at work