G1 vs G7

[Clem Bronkoski]

Getting into this LR stuff and reading a lot about ballistics I read where an individual should use the G7 BC when shooting long range type bullets. The manufacturers that list both G1 and G7 are few but I observed that the G7 BC is always significantly lower than the G1 BC. My problem is not all manufacturers list the G7 BC so how do you input data when you don't know what it is.
Example: I ran the same data through a ballistic calculator using the same BC for both G1 and G7 for my 300WM load with Barnes TSX bullets and even at 300 yards I noticed a difference and this difference became wider as the range increased to the point that at 800 yds it was 18.1 MOA for G1 and only 13.5 for G7.
That's a drop difference of just about 37" !!
Looking at some of the bullets that list both I took 60% of my bullet's BC and plugged it in and came closer to the G1 data.
Is there a way to more accurately calculate the G7 BC because most of the time I shoot without a spotter and getting close to my target is critical. I may be able to identify a hit 3' off depending on conditions but not always.
Am I splitting hairs here or not. I realize I have to do my own shooting to determine what my actual drops are at varying distances.

 

[med358-boise]

I suspect you have made an input error. For example, I just ran the Barnes 168 TSX using the G1 published value of .404 with a velocity of 3100 and with a 100 yard zero and it is -160.5 inches and -19.2 2 MOA @ 800 yards. Running the same calculations using the G7 of .200 (from the JBM ballistics library) it is -159.5 in and 19 MOA of drop. That is 1 inch and .2 MOA of difference.

Just FYI G1 & G7 are different formulas so you can't compare the numbers two each other.

 

[std7mag]

@Clem Bronkoski,

Which TSX are you trying to figure out?

I can run through Strelok Pro to see if they have a G7 listed.
Of course your velocity, twist rate and elevation help too.

As @med358-boise pointed out, they are figured differently.
For long range the G7 is going to be your more reliable data.

 

[Clem Bronkoski]

@Clem Bronkoski,

Which TSX are you trying to figure out?

I can run through Strelok Pro to see if they have a G7 listed.
Of course your velocity, twist rate and elevation help too.

As @med358-boise pointed out, they are figured differently.
For long range the G7 is going to be your more reliable data.

thanks .308" 180 TSX, 1:10, 3105 fps., 1.94" sight ht

 

[TucsonFX4]

Use G7. G1 is for flat base bullets and G7 is for the more aerodynamic bullets with boat tail, etc.

It stands to reason that even with a super high G1, the flat base profile will not be as aerodynamic at longer ranges, thus giving bad drop data at further distances.

I suggest always using G7. If the manufacturer is living in 1960 and wants to drive the hype train with only providing G1, cut the number in half for a starting point and call it G7 (or use what someone else has calculated for that bullet, but it will still just be a starting point for your barrel).

 

[Clem Bronkoski]

I suspect you have made an input error. For example, I just ran the Barnes 168 TSX using the G1 published value of .404 with a velocity of 3100 and with a 100 yard zero and it is -160.5 inches and -19.2 2 MOA @ 800 yards. Running the same calculations using the G7 of .200 (from the JBM ballistics library) it is -159.5 in and 19 MOA of drop. That is 1 inch and .2 MOA of difference.

Just FYI G1 & G7 are different formulas so you can't compare the numbers two each other.

I was using the Hornady ballistic calculator and with G1 @ 800 yds with .450 bc my MOA adj is 18.1. Using the same BC and G7 its 13.5.
If I reduce the BC to .27 at 800 yds my adjustment is 16.3. My initial thread was using the same BC for G1 and G7 which is why the difference.
I ran your number on the Hornady calculator and got 19.9 for G1 and 20 for G7. Remember the 180 has a higher BC than the 168.

 

[TucsonFX4]

 

[TucsonFX4]

Also, you can't use the same number for G1 and G7. Like I said, cut the G1 in half as a rough estimate for G7 number.

 

[biednick]

G7 is generally more accurate for modern boat tail bullets. BC varies with velocity, and G7 varies less with boattail bullets than G1.

G1 and G7 values are vastly different, and the calculation changes depending on which ypu choose. You can't take a G1 BC, tell your calculator it's G7, and get anything remotely close to correct.

There's no way to accurately calculate BC without equipment that's prohibitively expensive. You need a radar system that can track a bullet over several hundred yards. You can get closeish by taking half the G1 value, but you'll have to spend a good amount of time truing you the value to get something close.

 

[med358-boise]

thanks .308" 180 TSX, 1:10, 3105 fps., 1.94" sight ht

G1 146.0 inches drop 17.4 MOA
G7 143.3 inches of drop 17.1 MOA

You should familiarize yourself with JBM ballistics

 

[Clem Bronkoski]

G1 146.0 inches drop 17.4 MOA
G7 143.3 inches of drop 17.1 MOA

You should familiarize yourself with JBM ballistics

thanks med. That's really close to the .27 BC I used as a guesstimate for the G7 BC. I backtracking your numbers the G7 BC is .26

 

[TucsonFX4]

G7 is generally more accurate for modern boat tail bullets. BC varies with velocity, and G7 varies less with boattail bullets than G1.

G1 and G7 values are vastly different, and the calculation changes depending on which ypu choose. You can't take a G1 BC, tell your calculator it's G7, and get anything remotely close to correct.

There's no way to accurately calculate BC without equipment that's prohibitively expensive. You need a radar system that can track a bullet over several hundred yards. You can get closeish by taking half the G1 value, but you'll have to spend a good amount of time truing you the value to get something close.

I'll give a little plug for Eagle Ballistics (@Barbourcreek) for truing BC. Very easy and fast, as long as you have access to shooting at 600+ or so.

 

[med358-boise]

FYI JBM is showing the G7 as .229

 

[med358-boise]

I would add that all published BCs are just that published. BCs will be slightly different in every rifle based on velocity and some other factors. Therefore to be very accurate - you zero @ a distance usually 100 then shoot and document actual drop several 100 yards further at that distance. Then input actual drops and either manually manipulate BC till the drops match or some programs will calculate the "true or correct" actual BC automatically.