Anyone find that their custom drag not as close as the G7?

[Mr.Knowit all]

I was out shooting (3 different rounds) and I was over shooting about .75 MOA at 608 yard and about 1.5 MOA at 1764 yard targets. I was using my Kestrel Elite Ballistics and I switched to the G7 that Litz has in his book and it was on the money. I double checked my velocity and zero and it was good. So why was I over shooting? Anyone else have that issue or anyone else surprised that the CD was not as precise as thought?

 

[Ballisticboy]

A Cd drag curve is only as good as the methods used to derive it. It is my experience, after years of using all types of radars and devices, that using a fixed head muzzle velocity type radar simply does not cut it. We ended up using a million dollar tracking doppler radar but even that was replaced by a much more powerful multi million dollar radar which had to be operated by a skilled, experienced operator. The powerful radars are needed to obtain a full track of the bullet throughout its entire flight regime. Trying to obtain drag coefficient values at lower speeds by using reduced charges and muzzle velocities does not produce the correct values.
The analysis also has to be carried out using fully validated software, developed specifically for the purpose, which takes into account all of the variables.
It is extremely easy to get what appear to be perfectly acceptable results only for them to turn out to be rubbish later on after full analysis.

 

[Mr.Knowit all]

Wow!!! You said all I was thinking. I understand that a BC given is an average or only good for a specific muzzle velocity but muzzle velocity is always changing. I have seen people try to reinvent the wheel and MANY ideas sound great and make sense but do not translate into empirical results when applied.
All you Litz fans and want-a-bees (I am one) don't get all hurt or upset... I was just asking a question. If anyone else had their CD profiles a little off as well.

 

[dcali]

I think shooters expect more precision from BCs (and drag curves) than is reasonable. It's just not reasonable to expect BCs to be accurate to better than a few percent, especially when lot-to-lot variations are accounted for. So take a typical .300 G7 BC - 3% of that is .009. When you think about it, it doesn't really even make sense to report BCs to three decimal places - it gives the illusion of precision that isn't really there.

As for Cd curves - Ballisticboy has summed it up well, and I would defer to his expertise. One thing I would add is that the region of the curve where custom Cd curves for typical bullets diverge from the G7 curve is primarily in the transonic region, which where Cd uncertainty is high. So when you get into ELR, where you are going to go sub-sonic, it really is asking quite a bit if you want precise drag models.