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Thread: .308 muzzle velocity

  1. #1

    .308 muzzle velocity

    Hi there,
    i have a .308win which I intend to get the thread cut off and then have the rifle re-crowned and threaded to match a different thread.
    the barrel is currently 20 in so guess after the work is done the barrel might be approx 19 inch?
    would this put me in danger of not being deer-legal due to insufficient velocity etc?
    i don't have access to a chrono to measure the current velocity.
    thanks for any help,
    mike

  2. #2
    As a very rough guide expect to lose about 50 fps per inch of barrel you lop off. Only way to be sure is to use a chrono, though. The speed data in load tables or ammo packets tends to be based on 27" test barrels, which you're unlikely to have on a stalking rifle, so if that's the case, you can't be certain what MV you're starting out with.
    Speed doesn't matter per se in England & Wales directly, however muzzle energy does, which is derived from speed and bullet mass. There's a useful calculator here: Muzzle Energy Computer Interestingly, I just used that to work out how slow a 150gr .308 round would have to be going to fall foul of the law. It would have to be doing less than 2259 fps, which is pretty darned slow.

  3. #3
    Thanks for the info klunk

  4. #4
    I would think 50fps per inch is a bit high on average in a 308, it seem that the smaller calibres suffer more from taking off length, but different powders will perform differently depending on barrel length. Have a look at the table on this very subject in the following link.

    http://www.6mmbr.com/308Win.html

  5. #5
    +1 for jamross.

    Small cals and those using large charges of slow burning powder.

    Here s a slightly more accurate way of working it out.

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  6. #6
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    I raised this some years ago when the "fetish" for moderators became vogue.

    Apparently you have to go very very short indeed to fall below the velocity requirements. I think that the figure is 16". But...having said that....it depends what your starting velocity is with a "normal" 24" or 22" barrel.

    I'd say that 308 with a 150 grain bullet and a SLOW powder making 2,500fps out of a 22" or 24" barrel would be too close to becoming illega,l under Scottish velocity requirements, at less than 20".

    As a rule of thumb, in a rifle, drop 25-35 feet per second per inch lost from your original uncut barrel length figure.

  7. #7

  8. #8
    Quote Originally Posted by Sharpie View Post

    Very interesting- Potts really seems to know his stuff!

  9. #9
    Another thing is that if you reload Mike you might see little or no effect in real world terms - my 308 is a Blaser which I think has a barrel somewhere just over 21 inches and I have some reloads, just within book max, that give me 3000fps with a 150 grain bullet and no pressure signs plus many cases with lots of reloadings on them and they are still going strong. There was no skill involved on my part but I simply got lucky with the combination of powder, bullet and barrel length. I'd guess that with some care you could develop a load for the 19 inch barrel that would easily make 2700 and might make 2800fps if you get a lucky break like I did, that would have you matching the velocities quoted for factory ammo.

    For what it is worth I'm using a max charge of Reloader 15 behind a 150 grain Hornady Spire Point to get 3000fps. A Nosler Partition showed no pressure signs over the same load but wasn't accurate until I backed off quite a bit.
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  10. #10
    Quote Originally Posted by caorach View Post
    Another thing is that if you reload Mike you might see little or no effect in real world terms - my 308 is a Blaser which I think has a barrel somewhere just over 21 inches and I have some reloads, just within book max, that give me 3000fps with a 150 grain bullet and no pressure signs plus many cases with lots of reloadings on them and they are still going strong. There was no skill involved on my part but I simply got lucky with the combination of powder, bullet and barrel length. I'd guess that with some care you could develop a load for the 19 inch barrel that would easily make 2700 and might make 2800fps if you get a lucky break like I did, that would have you matching the velocities quoted for factory ammo.

    For what it is worth I'm using a max charge of Reloader 15 behind a 150 grain Hornady Spire Point to get 3000fps. A Nosler Partition showed no pressure signs over the same load but wasn't accurate until I backed off quite a bit.
    Thanks for the info, I don't re-load at the moment but might at some point in the future.
    Will be using factory ammo in the rifle for the time being so hoping that the barrel will still produce deer legal energy with that. I guess velocity only becomes a problem if I get lucky and come north of the border for some sika or reds.

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