Estimating Muzzle Energy

Discussion in 'General Airgun Chat' started by DeanB, Feb 7, 2019.

  1. RobF

    RobF Administrator Staff Member

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    :D
     
  2. cloverleaf

    cloverleaf Active Member

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    No worries - it's always (usually :p ) interesting to discuss this stuff - I find Jim's a great bloke to talk to on such subjects. Hello Jim if you're reading ;)

    Another couple of factors to chuck in to complicate matters are transfer port flow area (you often can't assume simple constant pressure then pure expansion once the valve has shut, as a smaller than ideal transfer port will choke flow and drop mean barrel pressure even when the valve is still open) and striker bounce. We can calculate how much air a gun uses "per shot", however this doesn't take into account that maybe only 60-70% of that amount is actually involved in pushing the pellet down the barrel; the rest being released once the pellet is long gone as the striker oscillates against the valve creating numerous subsequent valve actuations and "firing events".

    This brings us nicely onto Bob's Airwolf. As much as I'm no fan of the brand, one thing Daystate got bob on with their electronic striker system is its total elimination of striker bounce; making the guns very air efficient. it must also be remembered however that you can't simply compare two guns on pressure drop per shot, since cylinder capacity makes all the difference and in this case the Airwolf has a pretty significant (400cc?) cylinder hanging off the front.

    A crude efficiency yardstick I've used for a while is "bar.cc/shot" or even better "bar.cc/ftlb". Assuming Bob's Airwolf is doing around 11.5ftlb and has a 400cc cylinder, this would suggest an air usage, from 0.3bar/shot, of 0.3*400/11.5=10.43bar.cc/ftlb. Comparing this to a typical AA 400 series that gives say 80 good shots at 11.5ftlb over a 65bar pressure drop from a 215cc cylinder capacity gives (65/80)*215/11.5=15.2bar.cc/ftlb..

    This indicating that the Airwolf is using around 2/3rds the air per shot that the 400 series - not to be sniffed at!

    Of course with your equation for calculating the potential energy of the pressurised gas we can view these characteristics as actual efficiencies rather than abstract units that only make sense for comparison to each other.

    All good fun :D
     
  3. martin blue eyes

    martin blue eyes shoot, pray and hope

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    I always thought it is how many pages of the yellow pages the pellet goes through is how you estimate the muzzle energy !!!!!!!!!! well you learn something new everyday.
     
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  4. Nomads HFT

    Nomads HFT Active Member

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    To measure how much air was used each shot, a friend once attached a deflated balloon to the muzzle of one of his Daystates, lowered the balloon into a bucket of water (to make it easier to calculate the volume of air), and dry fired the rifle. The expression 'dry fire' is perhaps inappropriate in this case, because the rapid expansion of the balloon displaced much of the water out of the bucket.

    Indeed I am, Mike. Your posts are always worth reading.
     
    Last edited: Feb 8, 2019
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  5. cloverleaf

    cloverleaf Active Member

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    Grand - nice to find you on a commonly-frequented forum!

    That's an "interesting" approach to measuring the rifle's air usage - would it not just have been easier (and drier!) to have worked it out from the cylinder volume? :p
     
  6. Nomads HFT

    Nomads HFT Active Member

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    Where's the fun in that? :p
     
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  7. C.Eaton

    C.Eaton Confirmed Anschutz Nut...

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    Ok scrub that one then, I guess you’d have to subtract the barrel volume as well...
     
  8. DeanB

    DeanB Active Member

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    Here is a link to a paper describing a model of a springer air rifle....
    (PDF) Internal Ballistics of Spring Piston Airguns
    I think you might need to join ResearchGate to get access to the paper.
    The paper considers many of the factors identified in earlier posts and also some more.
    It also references other papers that have looked at modelling air rifles.
    Apologies if this paper has already been cited on STB.
     
  9. DeanB

    DeanB Active Member

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    And here is link to the same author's work on PCPs. Seven equations but they need to be solved numerically.
    (PDF) Internal Ballistics of PCP Airguns
    He has also published other work about air riles.
     
  10. RobF

    RobF Administrator Staff Member

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    The maths is beyond me.

    Is the answer 42?

    But I do know that if you can reduce valve bounce by only changing the valve side, it might affect the hammer bounce, but it definitely uses less air. What I don't know is if it changes the recoil over a time period, ie less over longer, or more over less time. Anyway intersting stuff!
     

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