Ever since one of you guys called me on the phone the other day (I think it was elkaholic) I've been thinking about this as an independent problem.
Then another case (dmgreene).
Then it hit me, I've seen this with the 6.5mm 140 VLD myself when helping a friend with a new rifle. With us, we started at 900 where the elevation was 'perfect', then we moved in. We shot at 800, 600, 500 and 200. At those ranges, we were high by 1/2 MOA (800), 1 MOA (600), .8 MOA (500) and 1 MOA (200). Another way to look at this is if we were zeroed at all the other ranges, we would have been about 1 MOA low at 900 (just like you guys) compared to the G7 predicted trajectory.
So that's 3 convincing cases of 'fishy-ness', I'm interested.
I went back to my measured drag for this bullet. It was tested on two separate occasions and there was quite a bit of scatter in the data which isn't good. However, the data indicated a reasonable match to the G7 curve.
I then entered the bullet geometry into the JBM drag calculator (McDrag based) and compared the 'computer estimated' drag curve to the measured points and G7 curve. The computer estimated curve matched very well to the G7 curve (with the .918 form factor applied). There are not interesting inflections in either the measured curve or the predicted curve anywhere in the velocity range this bullet would see over 1000 yards. In other words, there's nothing about the drag curve (measured or predicted) that explains this extra drop at 900 yards.
But the extra drop IS there.
The next possibility is that the bullet has a minor dynamic instability, like the .30 cal 168 grain MatchKing (meaning that the bullet flies with a very small 'cone-ing' motion). Dynamic instability can set in at any speed, not just transonic. A minor dynamic instability doesn't necessarily ruin accuracy, but it can affect drag, raising the actual drag of the bullet above what it would be if the bullet were flying perfectly point forward.
If this is in fact the case, I would expect to see the excessive drop at 1000 yards (and further). Anyone have drop data for 1000 yards (in addition to the good data you have for closer ranges)? I have data for 1000, but it's not well documented, so I don't have high confidence in it (never though I'd need those chicken scratches for anything like this!)
It would be really amazing if we discover that this bullet does have a dynamic instability, given it's incredible history of success in LR Hunting and competition!
I know Eric Stecker will find this interesting. He predicted months ago, a prophecy to the effect of: "Given the accuracy of these measured G7 BC's, it may be possible to diagnose problems based on a mismatch between predicted and observed trajectory instead of the old habit of doubting the accuracy of the predicted trajectory".
I hope I'm not getting ahead of myself here, but if there is in fact a minor dynamic instability with this bullet, I'm very excited to have identified it so we can make a great bullet even better!!!
Thanks for sharing your observations guys.
-Bryan
By the way,
What twist rates are you guys using?