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Thread: Barrel Length vs. Velocity

  1. #1
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    Barrel Length vs. Velocity

    I remember seeing posts on this before but can't find anything in the search so here goes..

    I know a longer barrel will produce more velocity (IIRC about 10fps per inch after 10 inches??) but I also recall something on finding an optimal barrel length, i.e. the point when the pellet stops accelerating and friction starts to slow it down...Is there a formula for this?

    Cheers
    Greg

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    Quote Originally Posted by Thegreg View Post
    I remember seeing posts on this before but can't find anything in the search so here goes..

    I know a longer barrel will produce more velocity (IIRC about 10fps per inch after 10 inches??) but I also recall something on finding an optimal barrel length, i.e. the point when the pellet stops accelerating and friction starts to slow it down...Is there a formula for this?

    Cheers
    Greg
    Springer I'm assuming as springer and pcp are different.
    It does depend on how the rifle is tuned.
    Generally I'd suggest an optimum barrel length between 14 and 16" for a factory spec springer with varying levels of tune.
    Skirtless reduced diameter piston very different and you're getting where minute alterations make massive differences
    As for specifics, get in touch with Bigtoe. He's the man
    B.A.S.C. member

  3. #3
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    Quote Originally Posted by Thegreg View Post
    I remember seeing posts on this before but can't find anything in the search so here goes..

    I know a longer barrel will produce more velocity (IIRC about 10fps per inch after 10 inches??) but I also recall something on finding an optimal barrel length, i.e. the point when the pellet stops accelerating and friction starts to slow it down...Is there a formula for this?

    Cheers
    Greg
    Barrel length can only add to MV if there is high enough energy left in the expanding air to make use of the extra length. For sub 12 ft.lbs about 55 cm is optimal. Beyond this length the air in the barrel has not enough energy to overcome the friction and the extra length will actually slow the pellet down. I am talking from experience as I have all three lengths, Carbine, 41cm and 56cm for my HW 100 and have tuned the rifle to all three lengths. The gain from carbine to 41cm is quite substantial and less so from 41cm to 56 cm.

    A.G

  4. #4
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    Quote Originally Posted by robs5230 View Post
    Springer I'm assuming as springer and pcp are different.
    It does depend on how the rifle is tuned.
    Generally I'd suggest an optimum barrel length between 14 and 16" for a factory spec springer with varying levels of tune.
    Skirtless reduced diameter piston very different and you're getting where minute alterations make massive differences
    As for specifics, get in touch with Bigtoe. He's the man
    Yes it's for a springer.

    Quote Originally Posted by lensman57 View Post
    Barrel length can only add to MV if there is high enough energy left in the expanding air to make use of the extra length. For sub 12 ft.lbs about 55 cm is optimal. Beyond this length the air in the barrel has not enough energy to overcome the friction and the extra length will actually slow the pellet down. I am talking from experience as I have all three lengths, Carbine, 41cm and 56cm for my HW 100 and have tuned the rifle to all three lengths. The gain from carbine to 41cm is quite substantial and less so from 41cm to 56 cm.

    A.G
    That makes sense. Is that figure obtained through testing different barrel lengths with a chrono? What calibre did you test?

    I'm sure I read a post where someone had done the maths. I think it'll be an equation of swept volume of the cylinder, air volume of the barrel, air pressure & the coefficient of friction between the pellet & the barrel. Not straight forward anyway..

    It'll vary between calibres, given an equal swept volume of the cylinder and varying air volumes in the barrel?

    I've got a 600mm barrel and want to work out the optimal (and minimum) amount to chop off!

    Cheers
    Greg

  5. #5
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    Interesting topic but I am mystified by all the theory,Theoben made the very effective Fenman with a 7.5 inch barrel,its very accurate and is equal to my HW77 for power,so does it really make any difference for the hunting gun,target maybe.

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    Quote Originally Posted by sarfarm View Post
    Interesting topic but I am mystified by all the theory,Theoben made the very effective Fenman with a 7.5 inch barrel,its very accurate and is equal to my HW77 for power,so does it really make any difference for the hunting gun,target maybe.
    Accuracy within the realms of the sub 12 ft.lbs does not really become a factor with barrel lemgth if the barrel is of high enough quality, LW barrels are used by a lot of decent makes of air rifles including Daystate and AA . The Fenman is a gas ram design I believe, either LE or HE and the power is the simply governed by the gas ram pressure and the length of the barrel. Alter the pressure high enough and it can probably reach 18~20 ft.lbs with a short barrel.

    A.G

  7. #7
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    Quote Originally Posted by Thegreg View Post
    Yes it's for a springer.



    That makes sense. Is that figure obtained through testing different barrel lengths with a chrono? What calibre did you test?

    I'm sure I read a post where someone had done the maths. I think it'll be an equation of swept volume of the cylinder, air volume of the barrel, air pressure & the coefficient of friction between the pellet & the barrel. Not straight forward anyway..

    It'll vary between calibres, given an equal swept volume of the cylinder and varying air volumes in the barrel?

    I've got a 600mm barrel and want to work out the optimal (and minimum) amount to chop off!

    Cheers
    Greg
    It is your barrel Greg and you may do with it as you wish but persoanlly I am not in favour of chopping barrels if they are of the choked variety unless it is possible to chop it from the breech end.

    A.G

  8. #8
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    I'm sure there was an article in one of the comics recently about use heat/cold to impart a choke on the barrel.

  9. #9
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    Quote Originally Posted by lensman57 View Post
    It is your barrel Greg and you may do with it as you wish but persoanlly I am not in favour of chopping barrels if they are of the choked variety unless it is possible to chop it from the breech end.

    A.G
    The barrel is currently a blank so it will be shortened from the breech end. I think 600mm is probably too long so didn't want to leave it as it is, hence I'm trying to establish the optimum length to trim it!

    Cheers
    Greg

  10. #10
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    There is no simple formula for the barrel length to give maximum muzzle energy for a given setup. The length required will depend on the pellet design and weight, the dimensions and specification of the piston and spring, the stroke length, the transfer port size etc. There are computer based time stepping simulations which will give a guess but I would not trust them to be sufficiently accurate to start chopping up barrels based on their predictions.
    All I would say is the amount of energy lost by having too long a barrel does not seem to be very large. Simulations I have carried out suggest an optimum barrel length of around 16 inches which is not surprising as the simulation was set up to give that value based on measured data. If you have a short stroke with a strong spring the length can come down to 13 inches. This is not to say you cannot get the energy with different barrel lengths and stroke lengths, it is just that you will not be at the most efficient combination in terms of muzzle energy.

  11. #11
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    It's just a guess but I'd expect it will differ between guns due to transfer port size & flow.
    It will be the relation between the volume of the bore per inch & the swept volume of the cylinder but the flow through the port will surely govern the rate of pressure build up behind the pellet and therefore the rate of acceleration.
    The optimum length being the point where the pellet reaches the required velocity, and that in turn will depend on the mass & friction of the pellet.

    Presumably a factory barrel length has to be a compromise between a heavy pellet not reaching max velocity & a light pellet being so fast the ME exceeds the limit.

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