help with drop field ... exball and field resoults

[300rum]

hi,

i got a strange situation...
i have a big difference on drop value from one day to another at diff distances.
2850fps, 300smk, 49f, 40%humidity, 24.75 pressure. 2mph from 3 clock
the same batch of reloading bullets, 0.5-0.75moa all day.
all the shooting are done from one slope to another over a small valley.
wind is blowing acros the valley, from 3 to 9 a clock.
so here is...
exball or jbm value are the same but real field value are off
407yrd - exbal 5.2moa, but field is 4moa
620yrd - exbal 10.75 but field is 10.5 at 8 degree cosine
975yrd - exbal 21.75 but field is 22.75 at 0 cosine

i double cheked everything, and this is drive me nuts...
any adviced, or help to solve this will be more then welcome.

[bigngreen]

Could you post scope height, zero range and height and what BC your using.
I couldn't get it to line up with the short and long but the 620 was close.
Maybe elevation also.

[300rum]

zero - 100yrd
scope - 2.345

alt . 5374ft
press - 838
i used multipe bc;
0.795 - 3150
0.780 - 2600
0.770 - 2400
0.630 - 2180
0.590 - 0

or g7 - 0.383

[bigngreen]

I guess I don't see the answer, I was wondering if you have verified scope adjustments as true. Also, were all shots cold bore or is your measurement off a grouping, are you sure about your range. I could get within a couple 10ths at the 620 and 975 but the 407 was 1MOA+ high. Hopefully someone will have some more ideas

[roaddog1m]

I was reading rather quickly so I may have missed it but, what was the temp on the day when you chrono'd your load?

Also, on the day that you were out shooting, had you laid your ammo in the sun or could they have been exposed to heat?


Tom

[Eaglet]

300rum,

I'm using G7 which I beleive is more accurate than using multiple BC's from G1.

That said, from my figured numbers, if I did it right, you know how that goes, garbage in garbage out; these are my conclusions:

1) From my number to your field results, the bullet impacted 6.9" low.
2) Again, following the same thinking, the bullet impacted 2.6" low.
3) Once more, the bullet impacted 6.64" high.

As mentioned above, seems like your ammo was in the sun; you can see how the bullet impact is moving gradually higher; or, you were shooting your groups without letting the barrel cool down enough... Once the barrel starts getting hot, if we leave the ammo in the chamber for little longer than a short time, it'll be enough to get higher muzzle velocities and impact higher as it gets warmer.

Then again, the differences are not that huge and shoulder pressure from group to group could do that.

If you chronographed the load, what kind of spread are you getting?

The numbers for vertical adjustment I got were: 5.4 moa; 10.9 moa; and 22.1 moa.

I hope it helps...

[300rum]

This happend all the time to this shooting location
See the photo bellow


I used swarowski range finder - multiple reading and got the same distance
The shots was at cold bore shot 4 moa at 400yrd, and the other 3 round at the same impact 4moa
Please note I used H1000 powder so no fluctuation on temperature.
Temp was 41F to 49F during shooting session., and the ammo was left on plastic case with the lead on, so no sun exposure.
3 shots, let the barel cool down, and no changes in elevation.
It sound strange but this can be the wind? the wind from the valley?
At the target and at the shooting location wind was 2-3 mph from 3 a clock.
Please note This happend all the time at 400yrd at different day, or different temperature.
I was in fall 2009, ian 12 2010, feb 4 2010, and all the time shooting at 400 yrd at the same target location (see the photo) over the small valley I get the same 4 moa elev. I know is off from 5.25Moa (JBM, Berger, Exbal) prediction, but I tried to get a logical explanation.
The fanny thing is if I go at 2200elev, on plain field and shoot at 400yrd, the 5.25 MOA match.
So by excluding one by one, I suspect the hidden wind from the valley bellow, can cause the almost 1.45 Moa difference.
Did you experience the same off value from shooting over a valley?