Why would my velocity go up in free recoil?

Discussion in '6BR, 6BR Improved & Wildcats' started by Falfan2017, Jul 7, 2018.

  1. Falfan2017

    Falfan2017

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    I was always under the impression that free recoil reduced velocity. However today I was testing shooting positions free recoil vs shoulder touching and trigger finger and nothing else.
    Load was 30.4 grain varget in 6br with a 105 hybrid Berger, lapua brass and cci 450. I know this is a fairly stout load and with my shoulder on the butt pad I was getting 2920 fps with standard deviation of 5.6 and max spread of 12 which I was fairly happy with. It created a .56" group and a .6" group at 200.

    When I tried free recoil to see what would happen with the exact same load I got an average velocity of 3040 with one even reaching 3085. Max spread of 87 and standard deviation of 40. Of course the group was worse but still about .75" at 200 yards. I can understand why the spread could be higher but how could it add so much velocity? Every shot with free recoil was over 3000 fps. Any ideas?
     
  2. BartsBullets

    BartsBullets Gold $$ Contributor

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    I’d say something happened with your chronograph.

    Bart
     
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  3. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    I think @dmoran has some data on different holds and velocity...
     
  4. dmoran

    dmoran Gold $$ Contributor

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    Tom Mousel tested the fallowing 3 rifle holds with 22-shots fired to each position (66-shots total):

    22-shot velocity averages (LabRadar)
    • 2672-fps - Free-recoil
    • 2670-fps - Light hand and shoulder contact
    • 2672-fps - Aggressive hand and shoulder contact
    Test was conducted using a benchrest rifle: Panda action, Krieger barrel, chambered in 6mmBRA with fire-forming loads
     
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  5. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Ok, maybe it was Tom that had data LOL
     
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  6. dmoran

    dmoran Gold $$ Contributor

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    It was me who put @tom to the task.... ;)
     
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  7. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    One thing to add, Tom loads his fire form ammo to the same precision as his match ammo. So Id expect this data to be as good as the lab radar is capable or reading.
     
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  8. TZaun

    TZaun Gold $$ Contributor

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    Does he have data for group size to along with?
     
  9. 6ShotsOr5?

    6ShotsOr5? Gold $$ Contributor

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    @Falfan2017, you didn’t mention what chronograph you were using. I think it was a Magnetospeed. In free recoil, the relative velocity you measure could be higher because the bayo is moving backwards. I have noticed with my own Magnetospeed that I get more consistent velos when I focus on tight shoulder contact and get a good forward lean into the buttstock to minimize the recoil motion. If you are not using a Magnetospeed, then I don’t know the reason you are seeing higher velos in free recoil. The Magnetospeed is the only chrono that I know of that is attached to the rifle and would show an increase in relative velocity due to momentary rearward velocity of the rifle.
     
  10. #40Fan

    #40Fan

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    ^^^ That makes sense.
     
  11. 6ShotsOr5?

    6ShotsOr5? Gold $$ Contributor

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    There may be more to the story though. I searched for more information and found some weapon recoil velocities for shoulder fired weapons in an old army report, table on page 5. http://www.arl.army.mil/arlreports/2012/ARL-CR-692.pdf I just scanned but I didn’t notice anything saying that they used a very loose hold to allow more rifle motion. The higher end of weapon rearward velocities were around 20 ft/sec, not 120+ ft/sec. Of course @Falfan2017 will probably tell us he was using an Oehler at any moment ... :D
     
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  12. alinwa

    alinwa

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    But regardless...... I LIKE THE WAY YOU THINK!! ;)

    I was just going to disregard this post as I shoot literally thousands of rounds over 9 different chronograph setups. But never would I even consider using a Magnetospeed so that thought didn't even cross my mind.

    nice
     
  13. Falfan2017

    Falfan2017

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    No it was a Caldwell set up about 10 feet down range. It's been reliable for me so far. I know it's not a top of the line chrono but I have no reason to believe it was malfunctioning. Had been testing loads with increasing powder charges and the increases in velocity it was showing were consistent with what you'd expect with 2 different rifles.
     
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  14. 6ShotsOr5?

    6ShotsOr5? Gold $$ Contributor

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    We will have to keep digging then, it’s a good mystery. Try the same test with different shooters and split samples from the same bullet lots. Try the free recoil first, then the firm grip. I agree with Bart about the chrono, see if it’s repeatable.
     
  15. Falfan2017

    Falfan2017

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    Got the same result a month ago. Huge extreme spread and higher velocity although not as big a difference. At the time I chalked it up to imprecise weighing as I was using a cheap scale at the range. However this time I loaded these to the kernel with a sartorius so I know the charges were consistent. Doesn't matter too much as my free recoil groups are consistently worse than shouldered. Just curious as to what's going on.
     
  16. Curious

    Curious

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    2920fps for 30.4gr of Varget sounds about right, I dont think that charge could ever give you 3040-3085fps under identical conditions.

    In a 6BR case to see a 160fps speed increase you would need to use in the region of 2gr more powder and you would have serious pressure - I would say your readings are suspect.

    In my experience a Caldwell chrono is about as trustworthy as Donald Trump.
     
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  17. bkluke

    bkluke

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    Did you load all your loads at the same time? Or were they exposed to direct sunlight? Anything that you can think of that would spike velocities?
     
  18. expiper

    expiper

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    the ambient temp. prolly went up and heated up the ammo,,,Roger
     
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  19. Falfan2017

    Falfan2017

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    I make sure to keep my ammo in the shade. The gun was at its hottest as it was the last group I fired but I'd only done 25 rounds over a half hour or so and it's got a straight 1.25" barrel so it wasn't too hot.
     
  20. WyleWD

    WyleWD Gold $$ Contributor

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    If you have a reflective type of chronograph. Sun angle, cloud cover, partly cloudy, inconsistent shade, all contribute to accuracy and repeatablility of your readings. The angle of the bullet's flight path relative to the sensors will make a bigger difference than some might imagine too. A few years ago I had a 6xc that was giving me fits with different readings, long story short the rifle was ok but my attention to detail in setting up the chrony set up left something to be desired. Maybe this link will help. WD http://precisionrifleblog.com/2012/...ctical-tips-to-increase-accuracy-reliability/
     
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