Trajectory Question

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by dwm, Nov 13, 2005.

  1. dwm

    dwm Well-Known Member

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    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
     
  2. Ridge Runner

    Ridge Runner Well-Known Member

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    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
     

  3. dwm

    dwm Well-Known Member

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    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.
     
  4. goodgrouper

    goodgrouper Well-Known Member

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    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.
     
  5. royinidaho

    royinidaho Writers Guild

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    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. /ubbthreads/images/graemlins/confused.gif

    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.
     
  6. Guest

    Guest Guest

    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.
     
  7. sierra22

    sierra22 Well-Known Member

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    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
     
  8. jb1000br

    jb1000br Well-Known Member

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    Was the trange flat or bumpy, valley in the middle?

    You most likely got lift due to the terrain and the wind hitting it and heading UP lifting the bullet -- BTDT...

    JB
     
  9. dwm

    dwm Well-Known Member

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    The range is south of Houston, TX, pretty much flat as it gets. You have to shoot past the 100 yard berm to get to the 200 yard line. The wind was blowing into the back of the 100 yard berm directly toward the bench. I don't have wihd flags but would guess that there were big eddy's coming off the 100 yard berm into the 200 yard line of fire.

    I was surprised Saturday to see that I was hitting higher at 200 than at 100, still getting nice groups though ....
     
  10. royinidaho

    royinidaho Writers Guild

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    I'd figure your're getting lift when the wind hits the back of the berm and shoots up. That would be pretty much like shooting across a canyon up here, you'll get a thermal somewhere between you and the target and shoot way, way high.

    I've watched hawks grab one of those thermals and gain altitude much faster than a fella would expect.

    Reasonable /ubbthreads/images/graemlins/confused.gif
     
  11. Brown Dog

    Brown Dog Writers Guild

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    I would guess something quite different...what are you using as a rear rest (and front rest for that matter) ...inconsistent rear rest technique/ poor selection of rear rest 'material' could change MPI from one occasion to the next....you'll still be able to shoot great groups; they just won't be in the same place each time...which could lead to apparently 'odd' ballistics if your rest/technique causes different vertical movement of the rifle in recoil from occasion to occasion..haven't explained this well -in a rush- hope it makes some sense!......I learnt almost same thing the hard way! /ubbthreads/images/graemlins/smile.gif Could never work out what was going on ballistically and was blaming my scope until a friend (far more experienced than me)told me to change my field expedient rear rest...problem solved. Hope this may be of help.
     
  12. dwm

    dwm Well-Known Member

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    Went to the range again yesterday, no wind this time. Also 65 degrees, YAY! To answer Brown Dog's question, I have been shooting off a Harris bipod with a normal benchrest rear rest.

    So with no wind I get 1.5 inches high at 100 and 1.28 inches high at 200 yards. Works out on the ballistic program if velocity is 3500 fps and the zero is at 245 yards. I did chrony them and and got an average of just over 3450 fps. (Also shot my first 1/2 inch 3 shot group at 200 yards.)

    Anyway thanks for all the ideas and help.
     
  13. long range erik

    long range erik Member

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    isn't it the other way

    a head wind will cause the bullet drop not raise because the wind is causing friction on the bullet causing the bullet to lose velocity making for a low hit on the target ect
     
  14. Boss Hoss

    Boss Hoss Well-Known Member

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    With respect to wind read and learn....

    Bench technique is also Critical!!

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