Elevation Dope

[liltank]

Okay, here is my dilemma! I have shot at 700 yards and 400 yards with my .308. My muzzle velocity is an average of 2612 with a 168 A-Max. I have been using a load of 48.0 grns RL17 with Wolf LR Primers. Everything seems to be spot on for grouping.

Here's the kick in the pants. On the day 12/30/09 we shot 700yrds I needed 22.25MOA up and 6.5MOA to the left. Conditions were less than desirable for this type of shooting but we had to deal with what we had. Temp 24, Baro 28.880, Humidity 71%, Elevation 978, overcast.

Wind conditions, 10-15 head on at the bench with an avg of 12. At the target left to right 8-12mph.

On 1/13/10 temp 29, Baro 28.77, Humidity 63%, Elevation 1079, slight mirage, winds right to left at 90 deg. 410yrds= 8.25MOA and Wind@10mph=2.75 dead on shooting a solid 1MOA group.

Now when I punch these numbers into my balistics program I can't get them to match up. My question is, will a strong head on wind force my bullet to drop requiring more elevation compared to a calm day? The second problem is that for 410 yards I have to leave the BC at the suggested factory BC and drop my velocity to 2600fps from the above avg. of 2612. The numbers are almost spot on. But in those heavy winds, I am 2MOA shy of what I needed to get successful hits at 700yrds. Can somebody help me here?

Dumbfounded,
Tank

 

[muleyman]

I'm not a scientist or a ballistics expert but an example comes to mind is a vehicle (poor example). When driving in a headwind it takes more gas to maintain the same or consistent speed and therefore you get less economy ( in your case drop). Vice verses in a strong tail wind. Sorry about the limited I.Q. answer/example but made sense at the time..........

 

[BillR]

When I punch it into my Sierra Infinity program it comes up with 22.1 with the perimeters you gave and showing a headwind of 10 MPH so I don't think your drops are off its the program you used didn't account for the headwind because when I punched in the headwind it changed it to match your actual drops pretty closely.

 

[MontanaRifleman]

Tank,

A 10 mph wind is equal to about 14.6 feet per second, not enough to affect your drop that much. You could be experienceing some some down draft somewhere? I've expereinced some elevation changes with shooting in head and tailwinds also. Hard to say what all is going on. Shoot on a calm day and see what happens.

Cheers,

Mark

EDIT: 10 mph wind

 

[sniperjwt]

Tank,

A 100 mph wind is equal to about 14.6 feet per second, not enough to affect your drop that much. You could be experienceing some some down draft somewhere? I've expereinced some elevation changes with shooting in head and tailwinds also. Hard to say what all is going on. Shoot on a calm day and see what happens.

Cheers,

Mark

+1 to what montanarifleman said

 

[liltank]

Well just learned something about my program that I didn't know about before. We do have an input for a head wind. I was under the understanding that straight on or directly behind wind values equaled zero. This is obviously true for left and right windage, but never even though about how it would effect the flight for straight flight. I guess I can say I just learned something. My cousin used my same program and punched that information in and it came up to what it should have been. Problem solved, thanks guys.

Tank

 

[Russ Hatch]

http://www.longrangehunting.com/for...-changes-our-vertical-impact-fact-myth-51306/

 

[MontanaRifleman]

Tank, are you saying that programing in a 10 mph headwind causes a 2MOA elevation change @ 400 yds?

 

[liltank]

http://www.longrangehunting.com/for...-changes-our-vertical-impact-fact-myth-51306/

Thanks, Bryan is purtty smart feller!!! Glad we can pick his brain!

Tank

 

[liltank]

Tank, are you saying that programing in a 10 mph headwind causes a 2MOA elevation change @ 400 yds?

No, at 700yrds. The deflection when I shot at 400yrds was a perfect 90degrees with no prevailing head wind or tail wind. The program matches up if I should have known about the deflection of the stiff head wind forcing the bullet down. I would guess at 400 hundred I wouldn't see that much drop because it has much less Howitzer effect.

400=8.25 MOA up, 700= 20.25, but the 10-15mph head wind requires a 2 MOA lift to account for loss of velocity and wind pressure forcing on the nose of the bullet. This results in a number closer to 22.25 MOA.

Make sense?

Tank

 

[Michael Eichele]

I dont know if this adds any confusion or help to the mix.

Running the same bullet (168 AMAX in 308 win) at 2810 and using .490 for a BC I get drop charts to match my real world tests between 300-900 yards very well EXCEPT between 550 and 650.

300 is zero, 400, 500 and 550 are dead on when corrected as per the programs. Now at 600 yards corrected for as per any software, my bullets strike 5" high. Once I get out to 700, 800, 900 yards the software predicts drops flawlessly again.

Dont ask why, I have never been able to figure it out or correct it by switching to G5 or G7. I just know that between 550 and 650 to aim a bit low. I have yet to see where the load is at 1K.

Sorry if this just adds futher confusion.

M

 

[mikey_weaver]

head and tailwinds can affect your point of impact up and down another thing to think about is when your bullets transitions from supersonic to subsonic your b.c. can vary a little

 

[rscott5028]

another thing to think about is when your bullets transitions from supersonic to subsonic your b.c. can vary a little

...that would occur at 1240 yards with this load.


Nonetheless, I think what you may be referring to is that the bullet destabilizes as it goes transonic?

Either way, trajectory can be modeled with a couple of BC's within different velocity ranges. But, the factory G1 BC isn't far off from Tank's empirical data what I can tell using NFBal.