Range Report Extrapolating Ballistic Data

[bigjohn451]

I shot my first long range f-class match yesterday shooting 600yds. I adjusted based off JBM using Brian Litz's G7 BC's and shot 12" high. Once I moved down 2 MOA I was dead on and was fine.

Now I am trying to get my data closer for future shooting but the only way I get the same adjustment I actually used was to run my velocity up 230 FPS.

Is 1 Data point enough to think this new data will be closer? Or should I shoot at a different range to get a second data point.

Load: 7mmRemMag chonographed at 2700 FPS with 162gn Amax. JBM says the load is moving 2930 Fps to get the drop I observed.

 

[BryanLitz]

Re: Extrapolating Ballistic Data

BigJohn,

First, I think 2930 is a very likely velocity for that bullet from a 7mm RemMag. You might want to check your chronograph.

Second, have you verified your scope calibration? If your bullets really are going 2700, your scope might not be moving your point of aim up higher than you think, creating an illusion that the bullet is shooting flatter than it is.

Just a couple troubleshooting ideas. Good luck tracking down the cause.

-Bryan

 

[TimK]

Re: Extrapolating Ballistic Data

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">BigJohn,

First, I think 2930 is a very likely velocity for that bullet from a 7mm RemMag. You might want to check your chronograph.

Second, have you verified your scope calibration? If your bullets really are going 2700, your scope might not be moving your point of aim up higher than you think, creating an illusion that the bullet is shooting flatter than it is.

Just a couple troubleshooting ideas. Good luck tracking down the cause.

-Bryan

</div></div>

I had precisely this problem with a Leupy Mark 4. Dial up 30 moa, get 34 moa. Took me a while to figure that one out.

 

[bigjohn451]

Re: Extrapolating Ballistic Data

Will check both of these ideas. The load is only 63.1 of H1000 which is pretty low, 2700 was my target velocity. Will check scope calibration.

 

[KYpatriot]

Re: Extrapolating Ballistic Data

Hah! I have a similar problem - just added a thread for it.

However, when I got the scope I precisely measured out about 40 moa on a piece of cardboard at 100 yards and the scope tracked EXACT so unless it has changed that shouldnt be my problem. Perhaps i should check it again.

Read my thread and see if it describes the problem you are having...

 

[KYpatriot]

Re: Extrapolating Ballistic Data

BJ451 let me know what you figure out please...

 

[bigjohn451]

Re: Extrapolating Ballistic Data

Well i figured my problem out. My scope tracks 5 moa per 4 moa dialed. The 11 moa I actually dialed comes to 13.75 moa which is much closer to the 13.3 moa JBM called for.

Is there a way to adjust JBM to take into account the different tracking?