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Thread: fps question vs barrel length

  1. #1
    Distinguished Member 223's Avatar
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    fps question vs barrel length

    how much fps velocity would you expect to loose, if a load says 2900 fps in a 26 inch barrel but you use that load in a 20 inch barrel instead

    i know the only way to be certain is to chrono but anyone know roughly ????

    ATB

    Dave

  2. #2
    As a rough guide it's 100fps per inch.

  3. #3
    I have a 20" barrel and most manufacturer load bullet speeds are based on 24" or 26" barrels. Based on the chrono results I got on factory Norma I think you will find it that you will loss significantly less than 100 fps per inch (although I have heard that figure being given before as rule of thumb). My guess (there are a number of unknown variables) is you will lose between 65-80 fps so 390-480 fps overall.

  4. #4
    It really depends on the calibre.

    My 14.5" .243 shoots 75gr v-max at 3200fps. The same load through a mates 22" barrel is 3600fps.

    You are looking at 50fps per inch where as .308 you are looking at 25fps per inch.

  5. #5
    I was told pretty much the same as scotch_egg regarding the 243
    Al

  6. #6
    In Shooting Times of 16th August 2007 (page 24) there was an article written (I think) by Pete Moore. He made practical tests with a .308 and a .223 by progressively shortening the barrels and measuring the MV over the chrono. He compared it with the computer generated predictions of 'Quick Load'. For all practical purposes the results were very similar.
    Practical test for .223 gave 24'' 3,402 fps mv, whilst 20'' gave 3,253 fps mv and 18'' gave 3,185 fps mv. Using a 52 grain bullet.
    The .308 gave 24'' 2,905 fps mv, whilst the 20'' gave 2,851 fps mv and 18'' gave 2,748 fps mv. Using a 150 grain bullet.
    Reducing from 24'' to 18'' the computer prediction was a loss of 194 fps whilst the practical test showed a loss of 217 fps for the .223. Difference 23 fps.
    The .308 predicted loss was 189 fps whilst the actual was 157 fps. Difference 32 fps.
    It was after reading the article that I had my Sako 75 .243 shortened to 20 inches and ordered my Sako 85 .30-06 with a 20'' barrel.
    One cautionary note with the .243 is whether it will generate 1,700 ft lbs muzzle energy with the shorter barrel. Some factory loads, particularly 100 grain could be maginal as they do not meet the claimed mv when tested over the chronograph. I home load using a Speer SPBT 85 grains at approx 3,100 fps mv, so no issue for me on that score. I hope you find this useful.

  7. #7
    Uncle I think you are mistaken with regard to the author of the article, it was Bruce Potts. Pete Moore is the editor of Shooting Sports. The one is well qualified and respected the other is ..... well the editor of a magazine.
    It's the calibre of the shooter that counts not the calibre of the rifle.

  8. #8
    Quote Originally Posted by 8x57 View Post
    Uncle I think you are mistaken with regard to the author of the article, it was Bruce Potts. Pete Moore is the editor of Shooting Sports. The one is well qualified and respected the other is ..... well the editor of a magazine.
    8x57 I like that. Very asute.

    I would certainly take Mr Potts advice over that of Mr Moore's.

  9. #9
    .223,

    A quick piece that explains it mathematically:

    http://www.loadammo.com/Topics/October05.htm

    Regards

    Mike

  10. #10
    Thanks for the correction and my apologies to Mr Potts. I cut the results part of the article out and kept is but alas, not the name of the author. It certainly helped me in drumming up the courage to shorten my Sako 75.

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