Range Report G7 bc vs Custom Drag Models

[rybe390]

Hey everyone!

I just posted the same thread in Ballistic Calc discussion. Mods, if this isn't okay, please keep one of them wherever there'd be more traffic and discussion on the topic. Apologies if so!

I wanted to bring up a topic that I saw firsthand this weekend at my first match.

For reference, I'm shooting 6.5 grendel, Hornady 130 ELDM at 2370 fps, magnetospeed chronograph verified. G7 bc of .289 according to AB.

Anyways, I did this match with unverified data after doing a load workup that provided great accuracy for my gun. I knew exactly how fast it was going, and I have had great luck running AB without truing in the past, so, screw it, let's roll right?

Wrong. Past 500 yards, I noticed that my wheels were falling off. I was impacting way low at all distances, and I decided to run the custom drag model instead of the G7 for my load. The AB g7 for my bullet is .289. Well, after switching to the custom drag, I got a second round hit at 1,100 yards, so the drag curve is much more accurate. But, I've never seen this big a difference between G7 and the custom drag curve. Typically it's .2 mil at most, this was a full 1.7 mils of difference.

G7 .289 drop at 1,100: 13.2 Mils - Massive Miss
CDM Drop at 1,100: 14.9 Mils - Impact
G7 adjusted .243 g7 at 1,100: 14.9 Mils - Impact. Also same data for 600 and 700 as custom drag that provided impacts.


I know you can't use a g7 to model custom drag, but the g7 that lines up best with dope(like within an inch at most distances) to the custom drag curve is adjusted to .243 g7, coming from a .289 g7. The data with the custom drag and that new G7 line up with dope I saw at 600, 700, and 1,100 yards.

I could have fixed this by adjusting my muzzle velocity by a gigantic amount, using custom drag(which obviously was accurate), and matching BC to verified drop data(making my BC match my custom drag data). I know my MV is correct, so messing with MV didn't seem right. I'm just amazed at the massive BC difference.

Has anyone here ever experienced such a drastic difference in BC that required you to adjust the BC so significantly you began to question the validity? I ask this, because according to AB, the 123 eldm, and 130 berger hybrid, will both perform significantly better than my hornady 130 eldm. The Hornady 130 and the berger 130 both have the same G7, but massively different custom drag curves. The berger 130 has an almost identical custom drag vs g7, compared to the massively different hornady.

Am I seeing a degraded g7 bc in the hornady because it doesn't go slow too well? Can anyone help explain what I'm seeing here?

All that being said, I know have good dope and data to use for my rifle. I have verified impacts at 3 distances with known environmentals, so I'll be able to bend any calculator into giving me a solution for printed drop charts. It just freaks me out using such a low BC to get the right data, and that the custom drag and g7 were so different for this bullet.

 

[Deleted member 113831]

The published initial BC of the bullet is an average BC for a bullet launched at 2800+ fps. You are launching it at least 400 fps slower, so your BC is going to be considerably less at the muzzle.

 

[b6graham]

The published initial BC of the bullet is an average BC for a bullet launched at 2800+ fps. You are launching it at least 400 fps slower, so your BC is going to be considerably less at the muzzle.

can probably end the thread here

 

[_Windrider_]

Look at your original thread, AB told you the issue.

 

[MangoFett]

So with my .260 using the same bullet and using AB I had similar issues past 500 yards. Using the custom curve I starting impacting low and when I got out to 800 I was 1.2 mill off. Switched it to g7 and I was only .1 off at 1000 and .2 off at 1200 which is so close it's arguable if it's the data, me, or elements causing that to not be dead on so I consider that to be dialed in good enough for a match. Shot another full day to confirm data and it was spot on again.
Now when using drag models since you're not using a ballistic coefficient you need to understand the axial form factor is what's important here. In AB when you switch to custom curve it automatically throws it in at a 1.00. Your individual powder choice, your particular barrel and batch of bullets can effect enough that it's something you need to really tune if you want it to be spot on Ive been told. How much this effects it, I don't know as I haven't played with different loads on different barrels or even with different powders. What I do know is that it's seldom 1.00. it's a range between 0.90-1.10 and that is a big difference at even 600 yards. Hornady does a good job of explaining this in their 4DOF and they break down how to precisely get that axial form factor dialed in for your rifle and load.

 

[stevenc23]

can probably end the thread here

Except that Hornady publishes the Expanded BC values. So even at 2370 the published BC is much higher than .243

6.5mm 130 gr. ELD Match
0.554 G1
0.279 G7 for Mach 2.25 (2500 fps and above)

0.542 G1
0.272 G7 for Mach 2.0 (2230 fps)


0.535 G1
0.269 G7 for Mach 1.7 (1950 fps)

Assuming all info is entered correctly, zero is good, scope tracks true, etc, etc....................
It just means the the 130 eld in your rifle with that load lines up with a G7 curve with a BC of .243

or you can adjust Form Factor: