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Trajectory Question

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  #1  
Unread 11-13-2005, 08:17 PM
dwm dwm is offline
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Trajectory Question

I was shooting the 7MM RUM over the weekend and am confused by the results. I was shooting 140 grain Accubonds, published BC 0.485, at 3450 fps into sub 0.5 moa groups.

Saturday there was a 10 mph wind gusting to > 15 mph coming straight toward the bench. I was hitting 1 inch over POA @100 yard and 1.5 inch over POA @200 yards. (still going up?? No way ???)

Today there was a 5-10 mph wind gusting to 15 mph coming toward the bench again. I was hitting 0.6 inch over POA @100 yards and 0.6 inch over POA @200 yards. (flat??)

I can't get any ballistic program to match this trajectory.

Is the wind causing the bullets to hit higher than normal at 200 yards? I thought headwinds caused the bullet to hit low.

What do you guys think?

Doug
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  •   #2  
    Unread 11-13-2005, 10:31 PM
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    Re: Trajectory Question

    what was the temp? your velocity changes with the temp, if you leave ammo in the truck and it gets warm it can change things drasticly
    RR
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      #3  
    Unread 11-13-2005, 11:31 PM
    dwm dwm is offline
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    Re: Trajectory Question

    Thanks,

    Takes 10 minutes to get to the range from my house, had the ammo in an insulated bag on the way there. Temp was 86 both days, kept the ammo in the shade. Was careful and made sure barrel cooled between shots. Put the gun on the rack in the shade to get it out of the sun today, so it would cool faster.

    Was shooting 94.5 grains of Hodgdon Retumbo. Hopefully it is stable enough to go from 76 in the house to 86 at the range.
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      #4  
    Unread 11-14-2005, 12:02 AM
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    Re: Trajectory Question

    Shooting through a chronograph is the only way to truly tell what is going on. It might be shooting faster and faster as the barrel wears in or the copper gets layed down in the bore. Who knows. The chrono is ultimate decoder.
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      #5  
    Unread 11-14-2005, 12:09 AM
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    Re: Trajectory Question

    Ran numbers through some ballistics software and "forced" it to duplicate your second day drops.

    First days numbers are impossible to compute??

    The second day flat numbers would work if the scope were raised to 2.5" above line of bore.

    Must be something with the terrain and wind that's doing it. [img]/ubbthreads/images/graemlins/confused.gif[/img]

    Will be interesting to see results on a calm day, but 10-15 mph winds don't seem enough to cause those numbers w/a 140gr bullet.
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      #6  
    Unread 11-14-2005, 02:35 AM
     
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    Re: Trajectory Question

    Doug,

    If you were facing a slight downslope, and the trajectory was slightly closer to level because of the position of the targets, a head wind can cause your bullet to drift high.
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      #7  
    Unread 11-14-2005, 03:43 AM
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    Re: Trajectory Question

    dwm
    wind in your face causes bullet to climb
    wind form behind causes wind to fall

    same reason why aircrafts always try to land with wind in the face - max lift in case of aborted landing.

    ...but i'm surprised at the amount of lift you got
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