Ballistics question: Stability

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  • Yoshi

    Invictus
    Jun 9, 2010
    4,520
    Someplace in Maryland
    Shooting 6.5 Creedmoor... At 100 and 300yds the rifle shoots a predictable MOA. At 200yds, the group opens up a noticeable amount. Any reason other than shooter induced error?
     

    Scrounger

    Active Member
    Jul 16, 2018
    357
    Southern Maryland
    Not having shot nor loading the 6.5 Creedmoor I can’t speak from any personal experience with the caliber.

    However, when load testing for group size with 80gr SMK in .223 I test at 300 yards. The bullet doesn’t go to sleep till at least 200 yards. If you aren’t familiar with the phrase of going to sleep. Heavy for caliber, i.e. long bullets wobble quite a bit when they exit the barrel. Depending on the caliber it can take a couple hundred yards for them to settle down. As far as group size at 100 yards for your load it is hard to say.
     

    ted76

    Ultimate Member
    MDS Supporter
    Jan 20, 2013
    3,151
    Frederick
    At longer ranges even the slightest wind will have an effect on group size, and any flinch or jerking of the trigger will be compounded.
     

    OLM-Medic

    Banned
    BANNED!!!
    May 5, 2010
    6,588
    Not sure.

    I honestly don't worry about measuring MOA anymore. If it hits the steel target at long ranges, I'm happy. All of the little variables for getting a perfect group size will drive you nuts.
     

    Uncle Duke

    Ultimate Member
    MDS Supporter
    Feb 2, 2013
    11,667
    Not Far Enough from the City
    Yoshi, I've seen this too.

    Perhaps related, perhaps not. But something I've also commonly seen is for short range accuracy (100 yards) to be notably better with flat base bullets of a particular grain weight, than a boat tail of the same weight and general configuration. I'm not sure why, but yeah it happens.

    Maybe E. Shell or someone else has a handle on this and can shed some light.
     

    atblis

    Ultimate Member
    May 23, 2010
    2,011
    I would first determine if your results are statistically significant. Design an experiment, conduct the experiment, and then evaluate.
     

    E.Shell

    Ultimate Member
    Feb 5, 2007
    10,245
    Mid-Merlind
    1) Check position. Position changes between distances often changes the degree of precision. If you have a drastic change between positions when you go to various distances, this can be the cause. Oddly enough, if your position change is only slight, you may not be taking proper care in re-alignment that you might take with a big position change. Check your natural point of aim, verify rifle fit. I know you know this.

    2) Scope parallax. Verify parallax setting when changing distances. Don't simply re-focus, do the head movement check.

    3) Visibility issues. Are some targets shaded while others are in bright light? Timing changes can affect sunlight issues.

    4) Barrel heat. If you shoot 100, then immediately shoot 200, but have a setup delay at 300, heat buildup that could have been affecting your 200 yard groups may have dissipated by the time you start shooting at 300.

    A stability issue (in-bore yaw) typically manifests itself as increasing smaller groups (as expressed in angular measure) as the wind of flight pushes the bullet back in line. IOW, your 100 yard group can be 1 moa (1") while your 200 yard group can be 3/4 MOA (1-1/2") and your following groups maintain that 3/4 MOA on out.
     

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