Applied Ballistics: G7 BC Vs Custom Drag Model

[rybe390]

Hey everyone!

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.

 

[_Windrider_]

I’m not all that familiar with 6.5 Grendel but it sounds like you are shooting at a lower than what is normal velocity which would make your BC lower.

 

[onevan]

Hey everyone!

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.

When you input G7, it is valid only for one exact speed of bullet. With longer distances G7 value decrease. So G7 is not good and same for whole trajectory of bullet. But Custom Drag is value measured and predicted for whole trajectory of bullet. Is better to use Custom Drag then G7.

 

[Deleted member 113831]

Hey everyone!

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.

Published BC's are an average over a given distance at a certain muzzle velocity, usually 2800+ fps. Your Grendel is starting out 400 fps slower, so your BC at the muzzle will be considerably lower.

 

[DocUSMCRetired]

I spent a couple hours yesterday going over random bullets. It would appear that bullets BC was input incorrectly. The CDM data is correct however. I have fixed it in the server, and it will be updated once we get a new build of some other stuff out with the fix as well. The G7 BC of that bullet is 0.247 and the next library update will reflect that.

 

[Kobyashi72]

It looks like you have a discrepancy between the G7 BC in the AB Connect library and the Data published in "Ballistic Performance of Rifle Bullets" (3rd Edition)

The G7 average across the entire velocity range is .247 and .243 (2000 - 1500) .244 (1500 - below)

 

[DocUSMCRetired]

It looks like you have a discrepancy between the G7 BC in the AB Connect library and the Data published in "Ballistic Performance of Rifle Bullets" (3rd Edition)

The G7 average across the entire velocity range is .247 and .243 (2000 - 1500) .244 (1500 - below)

Yes, the BC transcribed in to the server was incorrect. It has been updated and is only waiting for me to push a button to send it out to everyone. Before I do that, we are checking a couple other things.

 

[Deleted member 113831]

I spent a couple hours yesterday going over random bullets. It would appear that bullets BC was input incorrectly. The CDM data is correct however. I have fixed it in the server, and it will be updated once we get a new build of some other stuff out with the fix as well. The G7 BC of that bullet is 0.247 and the next library update will reflect that.

Thanks Doc for being on top of it. Everybody makes mistakes, but the fact that you guys moniter and correct immediately is a credit to you.

 

[Kobyashi72]

Yes, the BC transcribed in to the server was incorrect. It has been updated and is only waiting for me to push a button to send it out to everyone. Before I do that, we are checking a couple other things.

Thanks for keeping on top of it Doc.

 

[rybe390]

Thanks everyone for giving me input on the g7 side of things, nice to learn that because I’m starting slower I’ll have a semi degraded BC.

And Doc, thanks a million for looking into this, sorry to be a pain in your ass by calling too. Glad to hear that something is coming of it though!!

I’m a fan of the custom drag, so far from what I’ve seen, it’s dead nuts accurate.

Also, holy crap on the 130 eldm BC being so low. The 123 has a higher g7, as does the berger 130, looks like I might need to switch bullets now. So much for my load development haha.

 

[DocUSMCRetired]

Thanks everyone for giving me input on the g7 side of things, nice to learn that because I’m starting slower I’ll have a semi degraded BC.

And Doc, thanks a million for looking into this, sorry to be a pain in your ass by calling too. Glad to hear that something is coming of it though!!

I’m a fan of the custom drag, so far from what I’ve seen, it’s dead nuts accurate.

Also, holy crap on the 130 eldm BC being so low. The 123 has a higher g7, as does the berger 130, looks like I might need to switch bullets now. So much for my load development haha.

The form factor is 1.079 (About 8% worse than the standard). You generally want to shoot bullets that are closer to 0.90 or below that if possible. For comparison and examples sake the Berger 130 AR Hybrid has a form factor of 0.924 which is roughly 8% better than the standard. The standard being of course 1.00.

 

[Mhorn727]

The form factor is 1.079 (About 8% worse than the standard). You generally want to shoot bullets that are closer to 0.90 or below that if possible. For comparison and examples sake the Berger 130 AR Hybrid has a form factor of 0.924 which is roughly 8% better than the standard. The standard being of course 1.00.

Can you adjust form factor with Sig Kilo 2400ABS?

 

[DocUSMCRetired]

Can you adjust form factor with Sig Kilo 2400ABS?

You can perform a DSF calibration which is similar but more precise.

 

[LawnMM]

When you input G7, it is valid only for one exact speed of bullet. With longer distances G7 value decrease. So G7 is not good and same for whole trajectory of bullet. But Custom Drag is value measured and predicted for whole trajectory of bullet. Is better to use Custom Drag then G7.

Right from Litz the g7 is averaged from 3000-1500fps, if you're within that window, it's accurate.

When you drop below 1500fps and begin getting into transonic is where things get wonky

 

[Steel head]

Right from Litz the g7 is averaged from 3000-1500fps, if you're within that window, it's accurate.

When you drop below 1500fps and begin getting into transonic is where things get wonky

I gotta say as of late the Berger BC has been pretty spot on in that range

 

[Jabot]

I'll take Berger over hornady anyday. Haven't had a Berger blow up yet.good work fixing bc doc.