Weird Error In Ballistic Calculation

[BLC3128]

I had the opportunity to test out my 7mm WBY magnum at extended range last weekend. I use JBM ballistics to calculate my dope... However while shooting I experienced a strange error in the calculation of my drop. I'm going to provide as much information as possible and I hope somebody can chime in, because I am now clueless.

DATA

Nosler 160gr. Accubond @3240 (10yds from chronograph)
Scope Elevation 1.561"
35 degrees , cloudy, wind (5-10 <----)
Elevation 1600' Pressure 29.7
Zero'd @ 275 yards
Shooting mountain to mountain (over a dry dam)

The ranges I shot were 435 yards, 575 yards, 700 yards, and 835 yards. The elevation data was very close for all of the ranges except for the 575 yard elevation data. I used other various free calculators and got the same results... I even played with the input of the ballistic data, muzzle velocity, zero, with various combos and the 575 yard data was always off...I'm stumped!

Range (SUGGESTED DROP) (ACTUAL DROP)
435 yards - 3 MOA - 3.25 MOA
575 yards - 6.3 MOA - 5.25 MOA
700 yards - 9.6 MOA - 9.25 MOA
835 yards - 13.5 MOA - 13.5 MOA

Thanks in advance for your input.

 

[elkaholic]

Without more data, its difficult to say but I'll just throw this out because I've had it happen. When shooting over uneven terrain (canyons etc) it is very easy to get upslope wind affects which sounds like it may well have been your problem.

 

[Barrelnut]

Is it possible you could have mis-ranged the 575 yard target? Maybe it was a little further than thought.

 

[338 dude]

Possibly different cosign angle at that one distance?

 

[cohunt]

what is your zero distance? sounds like wind over uneven terrain to me though

 

[BLC3128]

what is your zero distance? sounds like wind over uneven terrain to me though

Rifle is zero'd @ 275 yards.

 

[BLC3128]

Is it possible you could have mis-ranged the 575 yard target? Maybe it was a little further than thought.

I thought the same until I shot the same target with my .300 and my friend shot it with his .338 Lapua and the dope was perfect according to the data.

 

[BLC3128]

Without more data, its difficult to say but I'll just throw this out because I've had it happen. When shooting over uneven terrain (canyons etc) it is very easy to get upslope wind affects which sounds like it may well have been your problem.

That makes some sense because it was the morning (8am) and the thermals were rising.

 

[elkaholic]

I thought the same until I shot the same target with my .300 and my friend shot it with his .338 Lapua and the dope was perfect according to the data.

from everything you've said, i think its up draft

 

[338 dude]

Rifle is zero'd @ 275 yards.

Zero at 100 your ballistic app will take it from there

 

[dfanonymous]

Probably your zero being at 275. Another thing about those long zero ranges...if you're off on the distance...at all..it just adds to error that will compound at distance.

 

[muleystalker]

Up draft from terrain/dry dam, wind blows hits dam/berm and creates an updraft. I see it all the time. At what distance is the dry dam down range in relation to your 575yd target? I used to shoot in an old rock quarry and there were all kinds of bench and different levels, it was enough to drive you crazy, but in the end it was a great teacher of how the terrain affects your shot and it made you learn to read not just wind but terrain too.

 

[BLC3128]

Up draft from terrain/dry dam, wind blows hits dam/berm and creates an updraft. I see it all the time. At what distance is the dry dam down range in relation to your 575yd target? I used to shoot in an old rock quarry and there were all kinds of bench and different levels, it was enough to drive you crazy, but in the end it was a great teacher of how the terrain affects your shot and it made you learn to read not just wind but terrain too.

Muleystalker,

I shoot directly over the dry dam and the dry dam is roughly 105 yards wide. If you look at my drawing, The dry dam begins @ 250 yards and the steep opposing hillside begins @ 355 yards.

 

[elkaholic]

Hmmmmm, with that set up, the targets past 575 yards should be even farther off with poi unless the wind was only blowing when shooting at the 575 yard target?