bushnell 10x tactical

6.5x55fan

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Dec 23, 2017
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I bought one on these for my 6.5 creed. I guess I don't understand how to determine the number of clicks on my turrets, to compensate for trajectory.. I have a range that will go to 400.I will be using factory match ammo. I will zero at 100 yards. Any help I would appreciate it.
 

308pirate

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  • Apr 25, 2017
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    In order to better help you, it would help us to gauge your knowledge of the basics. If you can answer these questions honestly, we can start with an explanation that won't leave you even more confused due to assumptions on our part.

    1. Do you know what a minute of angle is?

    2. Do you know what a milliradian is?

    3. Does your scope's elevation and windage knobs adjust in minutes of angle or milliradians?

    4. What happens to the bullet's trajectory if you set the rifle perfectly level with the ground and fire it?

    5. Do you know the muzzle velocity of the ammo you plan on buying? If not, find out in the manufacturer's website. You need to know what it is.
     
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    308pirate

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    Ok, that's what I thought but wanted to be sure.

    Looks like we were typing at the same time so I threw up some more questions to see where we are as a baseline. Answer up and we'll jump right in.
     

    6.5x55fan

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    Dec 23, 2017
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    1. Do you know what a minute of angle is? yes

    2. Do you know what a milliradian is? not without googling it

    3. Does your scope's elevation and windage knobs adjust in minutes of angle or milliradians? moa

    4. What happens to the bullet's trajectory if you set the rifle perfectly level with the ground and fire it? I think it would rise but not sure

    5. Do you know the muzzle velocity of the ammo you plan on buying? If not, find out in the manufacturer's website. You need to know what it is. 2710
     

    308pirate

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  • Apr 25, 2017
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    OK, here we go.

    First thing that I forgot, we need to know the exact bullet (brand, model, weight in grains) that the ammo you will use comes loaded with. If you can find the ballistic coefficient for that bullet, great, if not we can help you find it.

    Now, to clear up your fundamental misconception about trajectory (and you're far from alone). A bullet fired from a rifle that has be set level front to back will NEVER rise. It's physically impossible since bullets do not generate lift. In this case the bullet leaves the gun at the same angle (0 deg) to the horizontal as the barrel and begins to lose altitude (it starts to drop) immediately as aerodynamic drag starts slowing it down and gravity starts pulling it down.

    When you spin your elevation turret counterclockwise (the "up" direction) the image of the target is shifted upwards in relation to the reticle. That makes it look like the rifle is pointed down below the center of the target (or whatever you want to hit) and makes you physically angle the rifle upwards (pivoting on your shoulder) to bring the reticle back on target. That up angle of the barrel is what gives the bullet its upwards trajectory followed by its downwards trajectory after it peaks. The bullet climbs (due to the elevated muzzle) and goes above the line of sight between your eyeball and the target then as it slows it begins to descend back down through the line of sight until it strikes the ground. By zeroing at 100 yards, the physics works out that the bullet will be dropping back down from the peak of its trajectory at just about every usable distance beyond that.

    We use ballistic calculators to figure out the angle of elevation of the muzzle that will give us a trajectory where the bullet crosses the line of sight on its way down at the exact distance where the target is located.

    To use a ballistic calculator you need several basic inputs:
    The ballistic coefficient of the bullet
    The length of the bullet
    The mass of the bullet
    The muzzle velocity of the bullet
    The vertical distance between the bore centerline and the centerline of the scope or the center of the rear aperture if using metallic target sights
    The distance at which you've zeroed the rifle
    The farthest distance at which you want to know your trajectory
    The increments in distance to target that you want to know your trajectory (every 20 yds, every 50, whatever)
    The atmospheric conditions on the day when you'll be shooting. To keep it simple we'll assume a standard atmosphere (59 F, 29.92 in Hg barometric pressure), with 50% relative humidity and 1000 ft altitude above sea level.

    Let me know if anything needs more clarification before I go on.
     

    308pirate

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  • Apr 25, 2017
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    Cool, you're ready to go here: http://jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

    Find that bullet from the pull down menu and it auto populates all the bullet parameters so you don't need to worry about BC, length, weight, whatever.

    Plug in the muzzle velocity and all the rest of the parameters and give it a whirl. Play around with it and hit us up with whatever questions you have.
     

    6.5x55fan

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    Dec 23, 2017
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    What does lead indicate or meon the ballistic chartan
     

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    308pirate

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  • Apr 25, 2017
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    One more thing. I have the same exact scope you have. I bought it about 7 - 8 years ago when I didn't know any better.

    As you mention, its adjustments are in minutes but it has a midot reticle. The dots in that reticle are spaced 1 milliradian apart center to center. If you see the splash from a miss on your scope and you use the reticle to measure how far off from your target you'll have to convert from milliradians to minutes of angle if you wish to dial your correction. You could use the reticle to hold off by that many mils though.

    Long story short, learn about milliradians (it's not that hard if you don't care for the mathematical derivation) and next time buy a scope in which the reticle calibration and the turret adjustments match. Either both in MOA or both in milliradians (mils).

    To be completely honest, radians are just another angular unit of measurement. It is not a metric unit, any more than degrees are an "english" unit of measurement.

    You can call out elevation and windage corrections in fractions of degrees called minutes of angle (actually it's correctly called minute of arc, and one minute of arc = 1/60th of a degree) or in fractions of a radian called milliradians (one milliradian = 1/1000th of a radian). Ballistic calculators and apps can solve the problem for you in either unit.

    And finally some basic conversion factors:
    1 minute of angle = 1.047 inches per 100 yards
    1 milliradian = 3.438 minutes of angle
    1 milliradian = 3.6 inches per 100 yards