Adjusting for Spin Drift/Scope Tilt

Discussion in 'Long Range Hunting & Shooting' started by HuntnID, May 21, 2013.

  1. HuntnID

    HuntnID Well-Known Member

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    So I need some advice...I went out shooting my Cooper 6.5x284 yesterday. The rifle is topped with a NF NXS 3.5x15x50 so I am confident in the scopes tracking ability. It was a perfect day to shoot because there was no wind.

    I was not confident in the scope being perfectly level so I brought my level and drew a level line on the target. I shot a group at 100 yards adjusted my scope 30 minutes up and shot another group. The top group was 1/2 minute to the right. I had forgotten to bring a wrench for the scope rings so I could not rotate the scope.

    I moved back and shot 500 yards and hit the center of my 4" target.

    I moved back to 700 yards, shot and hit about 5" to the right. I had not adjusted for spin drift but I later calculated that my bullet should have drift .4 minutes to the right when it really drifted about .7 minutes.

    I shot at 820 yards, I hit about 6.5" to the right. Spin drift calculation says I should adjust .5 minutes although it drifted about .75 minutes.

    My question is, should I rotate my scope a fuzz to bring the spin drift calculations spot on? Or should I adjust my spin drift outputs to make up for it?

    If I should rotate the scope, which direction do I rotate it?
     
  2. cohunter14

    cohunter14 Well-Known Member

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    Re: Adjusting for Spin Drift

    The first thing to do is fix the scope so it tracks vertically. Once you do that, you need to shoot again. Chances are that your SD numbers will be pretty close once you fix your scope.
     

  3. CB11WYO

    CB11WYO Well-Known Member

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    If your bullets are impacting to the right, then i'm pretty sure your scope needs to be rotated Clockwise. (as viewed from the rear of the rifle)
     
  4. Greyfox

    Greyfox Well-Known Member

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    There are two approaches for dealing with spin drift. One is the set your scope up so it shoots exactly plum. You then calculate SD within your ballistic solution and adjust your combined wind drift and spin drift. The second is to built the cant into your scope mounting to compensate. For a right hand twist it would cant right from behind the rifle. I personally prefer a plumb scope setting since wind is usually a variable for me. I assume you are using a scope level which woul be mandatory when building the cant into your mountont to compensate for SD.
     
  5. HuntnID

    HuntnID Well-Known Member

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    So, I plan on rotating my scope to be dead level. Since my bullets are hitting too for to the right as I go out, which direction do I rotate the scope?
     
  6. aroshtr

    aroshtr Well-Known Member

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    Since you have proven that you are hitting to the right as dial the scope (100yrd test), this will only add to spin drift error. With a right twist barrel you will always drift to the right. I assume you are using a level in your system, and the action you need to take is different depending on where the level is mounted.

    If you have a rail mounted, or level on your ring cap... Then you would need to make a slight counter clockwise (from shooter position with the scope). After a slight adjustment re-test like you did at 100 yards. With varying wind effects it is safer to confirm at 100 yards.

    If you have a scope mounted level... moving your scope without changing the level position will do noting to fix the problem. In this case you would need to rotate the level clockwise (from the shooter position) slightly.

    This will correct so you can either be dead on, or like greyfox said you can have your scope level set to automatically correct for you spin drift. Example: If your setup has .5moa SD at 1000yrds, and it takes 24moa elevation to reach 1000, then dial up 24moa and adjust the level so your impact is .5moa LEFT of the plum line on your 100yrd target.
     
  7. RTK

    RTK Well-Known Member

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    Right to make it go left and visa versa.

    Reticle Perpendicularity
     
  8. HuntnID

    HuntnID Well-Known Member

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    Thank you all very much for the help. I can't wait to make the adjustment. It'll really help me get better at doping the wind.