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Thread: Longer barrel = increased velocity ? Discuss

  1. #1

    Longer barrel = increased velocity ? Discuss

    Until yesterday I was sure that the longer the barrel the greater the velocity for a given powder/bullet combination. However talking to an RFD yesterday he said this was not the case (specifically about a 17 HMR). His reasoning being that the modern powders have all burnt within a short distance from the chamber. Is he correct ?

    Answers gratefully recieved

    Scrun

  2. #2
    Hi Scrun, Not in my opinion.
    I have two rifles in 6.5x55, a Sako of 23 inch barrel length and the stutzen of 19 inches. With Hodgdens, IMR and Vihtavouri powders, through the chronograph the stutzen is allways between 80 and 100FPS slower than the Sako.

  3. #3
    I would make only two points:

    1. What your RFD said may well be true relating to rimfire rounds, but I would never look to extrapolate that to any general rule for CF rounds.

    2. There is more bullsh!t talked per square yard in gunshops than anywhere else in the known world. The crap I have listened to while biting my tongue in such places woulg begger belief.

    As a general rule, for CF rifles, longer barrels mean more velocity, but only up to a point, maybe 30" or so. However, some rounds suffer from velocity drop off much more than others. The bigger the bore size relative to case size, the less impact shortening the tube will have. So a 308 will suffer rather less velocity loss than say a 270win.
    Brian.

    Just because you are paranoid, doesn't mean they aren't out to get you......

  4. #4
    it is not as relevant on rimfire as it is with centre fire (even smaller calibre ones)
    the volume of propellant in rimfire is very fast burning (lil'gun-esque if not exactly the same) and is probably as your RFD describes.
    its why many people shoot 22lr and even HMR with 12-16" barrels without issue or huge reduction in velocity

    it is rumoured to be 50-100fps loss per inch of barrel reduction (calibre and load dependant)

  5. #5
    .22 L/R burns completly in something like the first 16" so anything longer can actually, in theory anyway, start to slow down. When my old and slightly battered BSA Supersport Five was cut and threaded it was cut to 16 1/2" and then threaded so in theory should produce the best velocity out of the ammunition.

    Centre-fires are different beasts and a lot depends upon the burning rate of the powder. My BSA CF2 Stutzen with it's 20" barrel produces an impressive fire ball at the muzzle with most factory and recommended 270 Win handloads so I switched to using a faster powder, H-380 in this case, which has reduced this effect.

    The .17 MHR is based upon the .22 L/R so it probably follows the same rationale and burns in abut the same time frame. I am no ballistics expert but rifle powders have to be of a progressive burn or pressure spikes would be too high for safety. With 60grains of powder igniting almost instantly I doubt many rifles would survive for long before suffering damage. Bear in mind soem of the lager cases magnums use much more than 60 grains of powder.

  6. #6
    Thanks for the info. As always things appear not to be as black or white as they first appear

    Cheers

    Scrun

  7. #7
    Cant remember where but have read a simular theory relating to the .17hmr that the optimum barrel lentgh is 16" after which all powder has been burnt so in theory friction will slow it down,probably explains why all that crud falls out the end of my 18" barrel

  8. #8
    Quote Originally Posted by bewsher500 View Post
    it is not as relevant on rimfire as it is with centre fire (even smaller calibre ones)
    the volume of propellant in rimfire is very fast burning (lil'gun-esque if not exactly the same) and is probably as your RFD describes.
    its why many people shoot 22lr and even HMR with 12-16" barrels without issue or huge reduction in velocity

    it is rumoured to be 50-100fps loss per inch of barrel reduction (calibre and load dependant)
    Most of the reading I have done on this subject would suggest that 50-100fps per inch is actually a bit much, with some reference being made to only 30fps per inch. It changes though as the barrel is increased or decreased. It is not quite as simple as saying 30fps per inch across the whole barrel length if that makes sense.

    I also read that with .17HMR in particular, anything beyond 16'' offers no real benefit at all.

    If you look into it, as I did a while back, certain calibres because of their bore size lend themselves to short barrels while still retaining good velocity because of the powders they can burn. One that springs to mind was the 7mm-08AI. I don't know if I can find the article but it said that this was one of the few that could still break the 3000fps mark from a 20'' barrel while using hunting bullets around the 140grn-150grn weight.

    25-06 was one I recall being referred to as benefiting from a longer barrel (at least 24'') because of the slower burn rate powder that worked well in it. I practical stalking terms does any of it make any real difference? I personally would suggest not as an extra 100fps or so may only make a difference in drop of .1 or .2 inches at 200yds from a 100yd zero.
    Last edited by jamross65; 05-09-2011 at 16:01.

  9. #9
    There are a few other things to consider while we are at it.

    I cut my 260rem from 22.5" to 20.5" and it cost me 45-50fps total. That is measured on a chronograph before and after. 25 FPS is not a bad rule of thumb for the 308 cased rounds, though the 243 may fair a little worse.

    The 30/06 based rounds will be worse than this, I would have thought a 25/06 or a 270 would loose near the 50 FPS per inch mark. I would not want to own a 270 with less than a 22" barrel. I do have a short barreled 30/06 which is not too bad and gets within 100 fps of book velocity with a 20.5" barrel.

    Also, the relationship between barrel length and velocity is not linear. Cutting say a 308 from 26" to 22" will have a lot less effect than cutting it from 22" to 18", ie the shorter you go, the more velocity you will loose per inch of barrel.
    Last edited by Claret_Dabbler; 05-09-2011 at 16:08.
    Brian.

    Just because you are paranoid, doesn't mean they aren't out to get you......

  10. #10
    A super long barrel with reduce the potential velocity of the bullet. The frictional forces acting on the bullet will remain constant while the maximum pressure achieved reduces as the volume of space for the gases to fill increases during bullet travel to muzzle.


    my hypothisis anyway

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