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Velocity vs Barrel Length

Discussion in 'Rifle Country' started by Dr T, Sep 21, 2019.

  1. Dr T

    Dr T Member

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    While sitting in the departure section for Southwest at Houston Hobby today, my wife came up and tossed a magazine at me that she had found abandoned in a seat nearby. It was a copy of Guns&Ammo Precision Rifle Shooter 2019 Issue 2.

    Now, I will be the first to admit that I am a bit of a snob when it comes to firearms literature (I generally just read Rifle and Handloader), but I was pleasantly surprised by this publication.

    In particular, there is an article by Patrick Sweeney entitled "Inch by Inch" that publishes a summary of his findings for his study of the decrease in velocity for the 6.5 Creedmoor as its barrel is shortened from 24 inches to 7 inches. The velocities published are 10 shot averages for 6 different loads with bullet weights ranging from 125 gr to 147 gr. The data was collected with a LabRadar chronograph. While it does not have the 2nd order statistics (standard deviation and extreme spread), it is still one of the more complete studies of this topic I have seen in the popular literature and I commend the author on the discussion he provides.

    Since I have only eyeballed the numbers so far (and have not done any number crunching of my own on his published averages), the spoiler is that it looks like about 20 to 30 fps per inch. I will feel better after I can run some regression lines on the data.

    And, If it happens to be the magazine that was left behind by someone on THR, rest assured that it has fallen into good hands and I greatly appreciate it.
     
  2. Alaskan Ironworker

    Alaskan Ironworker Member

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    That roughly correlates with my two length test on 6.5x55. The rifle had its original 29.5” military barrel and was spitting out 140 grain bullets at around 2850 fps. I cut it to 23.5” and now it gets about 2690 fps with the same load in the same gun.
     
    stillquietvoice likes this.
  3. NIGHTLORD40K

    NIGHTLORD40K Member

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  4. Jim Watson

    Jim Watson Member

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    Phil Sharpe once sawed off a 31" barrel .30-06 with an early counter chronograph.
    Velocity change was not at all a smooth decline.
    I saw it in a Rifle or Handloader some years ago, so you might have it .
     
    Last edited: Sep 22, 2019
  5. jmr40

    jmr40 Member

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    This has been done countless times over the last 100 years. No 2 people will get exactly the same results because even with the same barrel length 2 different rifles can see over 100 fps difference. But there is a reason why most manufacturers have pretty much settled on 20"- 22" as the standard barrel length for most non-magnum cartridges. You don't gain much beyond 22" and you don't start losing much until you get below 20". Magnum cartridges need a couple more inches and there are always exceptions to the rule.

    Around 25 fps/inch is a number thrown around often., but taking an average doesn't always tell the story. When you start at 28" and cut 1" at a time down to 16" you may see 25 fps as the average. But the numbers above 22" may well only be 5-10 fps/inch. Once you get below 20" you could see 50 fps for 1". And it depends on individual rifle and the cartridge. My 18" 308 is only about 50 fps slower than my 22" rifles with some bullet weights, closer to 90 fps with others.

    Here is some published 6.5 CM data.

    https://rifleshooter.com/2016/02/6-...el-length-on-velocity-cutting-up-a-creedmoor/
     
  6. denton

    denton Member

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    As the propellant gas expands and pushes the bullet along, the pressure drops. So the effect of cutting an inch off a 29" barrel is much less than the effect of cutting an inch off a 20" barrel. So a straight line regression is probably not going to work out very well. You might try substituting Log(length).
     
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  7. Archie

    Archie Member

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    The idea of fps per inch of barrel length is dependent on expansion ratio. So comparing a .30-06 round to a .45-70 round is pointless and usually misleading.
     
  8. WelshShooter

    WelshShooter Member

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    Came here to post this.
     
  9. e rex

    e rex Member

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    Ironworker,
    I have the 6.5x55 model 94 carbine. Loading a 140 grain Sierra over 43 grains IMR4350 I get just over 2400 fps.
    Mine has the 17.5" barrel with the 1/2" sleeve pressed on to allow importation before 1968.
     
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