Why the change?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Elarski, Apr 30, 2003.

  1. Elarski

    Elarski Member

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    My Savage 10FP-LE2 is currently sighted in at 1 1/2 " high at 100 yds. using 168 gr. Fed GM Match ammo. Today at the range, I tried the same ammo in 175 gr., and the point of impact shifted way right (at least 2"). I can understand a change in elevation, but I'm puzzled by the windage change. Any ideas appreciated. [​IMG]
     
  2. Brent

    Brent Well-Known Member

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    If it isn't wind, it's most likely the barrel vibration difference as the bullet exits the muzzle that's causing it or maybe a scope mount issue has just popped up. It's been noted, using the "same" bullet, most of the barrels vibratation caused dispersion is vertical in nature and very, very little is horizontal. Using a different bullet, well, it's not too uncommon then.
     

  3. *WyoWhisper*

    *WyoWhisper* Guest

    The heavier bullet and different velocity will cause different barrel harmonics.. I'm pretty sure.. also the recoil is different too. Is your barrel free floated? the different nharmonics may have caused the vibrations to contact somewhere...just a thought....
     
  4. Cobra Kahn

    Cobra Kahn Member

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    Elarski
    Do you have any of that 168 Gr Match Ammo left? It would be interesting to see if it prints 2" right or if its still on zero. If the scope hasn't physically moved then I'd tend to agree with Brent & Wyo in the harmonics theory.

    Harmonics obviously play a huge role in accuracy. I've seen groupings migrate upwards of an inch or so by simply verying the powder charge, so it would not total flabergast me if yours migrated by 2" from changing bullet weights.
     
  5. Elarski

    Elarski Member

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    Cobra: After experiencing the drift with the 175 gr., I shot 3 more groups using some more 168 gr. out of the same box, and they grouprd exactly as previous groups of 168 gr..

    Side note: I used PMC 168 gr. Match rounds, and they group the same as the Fed. 168 gr. GM.

    The barrel is free floated. So far the 175 gr. ammo is the only other weight I've tried. [​IMG]
     
  6. glbreil

    glbreil New Member

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    I have shot a lot of rifles testing loads. I always load my ammo in 1/2 grain increments when working up loads. I have found that even with that little bit of powder diffence the point of impact changes significantly, not only up and down but to the left and right. It definantly has to do with the barrel vibration and the time when the bullet exits the barrel.
     
  7. H-BAR

    H-BAR Well-Known Member

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    Hey Cobra by that quote do you think that Stormin' Norman is saying the French are LAME????
     
  8. MAX

    MAX Well-Known Member

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    I think he meant that some things are best left behind anyway. [​IMG]

    FWIW on the subject above, I've seen a number of rifles that progress through horizontal to diagonal to vertical strings before settling down to a nice round wad on target. Can't imagine it's anything else but harmonic/vibration changes or in the case in question, bullet jump due to different BC and design of the bullet. I have three rifles that have demonstrated a tendancy to shift laterally with bullet weight changes. I'm not going into a dissertation on the subject as I don't understand it well enough to explain, but Robert McCoy does in "Modern Exterior Ballistics", and M.L. McPherson does as well in the Spring issue of Varmint Hunter Magazine. Hope this helps.