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Thread: Barrel length and velocity.

  1. #1

    Barrel length and velocity.

    Hi guys, most of the common reloading manuals I have are of American origin. Our American friends tend to use longer barrelled (unmodified) hunting rifles compared to what we tend to use in the UK.

    If you take 243 and 308 for example, the trend in the UK seems to lean towards 20 inch barrelled rifles. Most of the reloading manuals I have read have loads developed for the above calibres in barrels that are at least 2 inches longer and in many cases 4 to 6 inches longer!

    Logically, if we assume we are getting the same velocity that our American based reloading manuals are telling us for a certain powder charge weight, we are probably significantly off the mark, With the actual velocity being significantly lower. Obviously the only real way of testing this out would to use a chronograph to measure the exact velocity of each load produced. In reality this is not a piece of equipment that the average reloader is likely to possess.

    If we take the case of the 243 for example, it is my understanding that with a 20 inch barrel, some factory produced Ammunition struggles to make the UK legal minimum muzzle energy requirement of 1,700 foot pounds, that is the legal requirement for use on our larger species of deer.

    I would be very interested to find out the average reduction in velocity in comparison to reduced barrel length for a given load, compared to the length of barrel that the load was developed in. For example, for a load developed in or fired from a 24inch barrel, what velocity reduction is likely from a 20 inch barrel?... 150 fps... 200 fps maybe?

    I would be interested in your views on the above and it would be very interesting to hear from anyone who has actually carried out such a test.

    Many thanks.
    Last edited by paultap; 08-02-2015 at 17:31.

  2. #2
    Hi I have a Tikka t3 6.5 x55 with a 20 inch barrel my friend has a Sauer 202 same calibre with a 24 inch barrel same load 140 grain sst I get 2650 fps he gets 2825 fps

  3. #3
    Quote Originally Posted by stephentri View Post
    Hi I have a Tikka t3 6.5 x55 with a 20 inch barrel my friend has a Sauer 202 same calibre with a 24 inch barrel same load 140 grain sst I get 2650 fps he gets 2825 fps
    That's interesting , muzzle energies by my calculations work out as follows

    20 inch barrel gives 2650 fps = 2183 ftlb

    24 inch barrel gives 2825 fps = 2481 ftlb

    So thats a drop of 175 fps and 298 ftlb for 4 inches of barrel length, quite a significant drop.


    Incidently, here's the formula used to work out the muzzle energy -


    velocity (in fps) x velocity (in fps) x bullet weight (in grains)
    ------------------------------------------------------------------- = Muzzle E in ftlb
    450240



    example for 2650 fps using 140 grain bullet is as follows -

    2650 x 2650 x 140
    ---------------------- = 2183 ftlb
    450240
    Last edited by paultap; 08-02-2015 at 19:05.

  4. #4
    SD Regular Greener Jim's Avatar
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    To be honest that's a pretty good retention of velocity. It's only 43.75 fps/inch loss.

    More overbore cartridges such as 22-250 would do much much worse.

    If you compare two cartridges from the same 'family' such as 243, 308 and 358 the velocity drop per inch lost is less in the larger calibres as the cartridge is less overbore.

    Here is an interesting article where they chop a 308 barrel down from 26" to 13.5" and they get a 25 fps/inch loss
    The Truth About Barrel Length, Muzzle Velocity and Accuracy - The Truth About Guns

  5. #5
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    You should chronograph your ammo anyway I have two 6mmBR rifles both barrels are from the same manufacturer same length same action with the same load one is 50fps faster than the other
    so if your load is marginal it would pay to know exactly what the velocity is running foul of the law is not a good move.

  6. #6
    Just did a lot of " web research" into this myself and concluded that for most stalking calibres around 50 fps per barrel inch is a reasonable rule of thumb

  7. #7
    Oh and meant to say that most factory data is based on a 24inch barrel

  8. #8
    Loss of velocity per inch of barrel length is very dependant in calibre. An overbore calibre will loose much more than an underbore one and a heavy for calibre bullet will loose less than a light for calibre bullet. A 20" rem mag with a 140 grain bullet will loose heaps, not worth doing, but a 20" .308 using a 180 grain bullet will not loose too much.

    There is always a tipping point you cannot dip below. I wanted to build a 16" .308 tracking rifle but the loss of velocity/energy for those additional 4" was huge.
    So much to learn and so little time left

  9. #9
    SD Regular Greener Jim's Avatar
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    You can still be deer legal at 15" with a 308

    Have a gander at pistol data
    Set your sights on pistol reloading data | Hodgdon Reloading

  10. #10
    Quote Originally Posted by Polly View Post
    Just did a lot of " web research" into this myself and concluded that for most stalking calibres around 50 fps per barrel inch is a reasonable rule of thumb
    Thirty-five fps per inch has always been my 'rough estimate' number when used in a ballistically balanced round. Not always right but, hey! That's why everyone should own a chronograph. It cracks me up that a reloader can spend $$$$ on a bit of shiny, micrometer read out reloading gear but won't spend a bit of cash on an inexpensive chronograph.~Muir

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