Ballistics Question

[HuntClub]

7mm Sierra Game King - amazing groups out of my 7mm08. Using Digital Chronograph my initial velocity averages 2525 fps from a 20" barrell. Using the Hornady Ballistic calculator with BC at 0.430 and the drag coefficient of [G7] the FPS is good for a long way - still measure 1995 fps @ 600 yards. Using the drag coefficient of [G1} however, reduces the FPS quite considerably over that distance - measuring 1542 FPS @600 yards.

Can anyone verify what the FPS should be 100-600 yards? I'm not likely to ever shoot further than 400 yards but I want to have confidence in my FPS numbers from 100 and out.

Any insight will be greatly appreciated.

 

[dhagerty40]

is this for the 140Gr game king? I can run a table for you in the sierra ballistics program.

 

[std7mag]

Only way to really know is to shoot it.

 

[David Milisock]

7mm Sierra Game King - amazing groups out of my 7mm08. Using Digital Chronograph my initial velocity averages 2525 fps from a 20" barrell. Using the Hornady Ballistic calculator with BC at 0.430 and the drag coefficient of [G7] the FPS is good for a long way - still measure 1995 fps @ 600 yards. Using the drag coefficient of [G1} however, reduces the FPS quite considerably over that distance - measuring 1542 FPS @600 yards.

Can anyone verify what the FPS should be 100-600 yards? I'm not likely to ever shoot further than 400 yards but I want to have confidence in my FPS numbers from 100 and out.

Any insight will be greatly appreciated.

How are your groups at 300 yards? What are the drops at 200 and 300 with a 100 zero? You know the muzzle velocity so the drops are far more important then the velocities. Unless this is an academic exercise.

 

[HuntClub]

is this for the 140Gr game king? I can run a table for you in the sierra ballistics program.

150 gr Game King - sorry I left that important piece of info out

 

[HuntClub]

How are your groups at 300 yards? What are the drops at 200 and 300 with a 100 zero? You know the muzzle velocity so the drops are far more important then the velocities. Unless this is an academic exercise.

I don't have a 300 yard (nor a 200 yd) range i can shoot at.

 

[jpmimproved]

if I understand your question correctly, I would look at the shape of the bullet to compare whether a G1 vs a G7 coefficient should be used. Generally speaking, a G7 coefficient would be recommended for a pointed nose boat tail bullet shape. G1 coefficients are most accurate for flat based, more round nosed bullets. One note to keep in mind, there will be different coefficients for G1 vs G7. If I remember correctly, Sierra lists G1. Bison Ballistics lists the G7 at 0.220 for the 150 .284 gameking. I would expect the G7 to be much more accurate than the G1 (form factor of 1.2 vs 0.69)

for more info on G1 v G7:

G1 vs. G7 Ballistic Coefficients — What You Need to Know « Daily Bulletin

The better, up-to-date ballistics programs let you select either G1 or G7 Ballistic Coefficient (BC) values when calculating a trajectory. The ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. You've probably seen that G7 values are...

bulletin.accurateshooter.com

 

[ShaunBrady]

7mm Sierra Game King - amazing groups out of my 7mm08. Using Digital Chronograph my initial velocity averages 2525 fps from a 20" barrell. Using the Hornady Ballistic calculator with BC at 0.430 and the drag coefficient of [G7] the FPS is good for a long way - still measure 1995 fps @ 600 yards. Using the drag coefficient of [G1} however, reduces the FPS quite considerably over that distance - measuring 1542 FPS @600 yards.

Can anyone verify what the FPS should be 100-600 yards? I'm not likely to ever shoot further than 400 yards but I want to have confidence in my FPS numbers from 100 and out.

Any insight will be greatly appreciated.

The G1 BC is 0.430. When you switch the calculator to G7, you'll need to provide a G7 BC which will be about half of the G1 BC.

Oehler has written that inside 1000 yards, it doesn't make a lot of difference which BC you use. You will have to provide the correct BC for the what the solver is expecting, but the computed drops should be nearly the same. It's generally considered good form to use G7 with boat tail bullets, but it's of little consequence in the application you've described.

 

[dhagerty40]

ok, here is what Sierra ballistics program came out with. ignore the path and wind columns as no gun info has been entered. Sierra also uses different BC's depending on velocity and also they have no G7 data for this bullet.

 

[Coyotefurharvester]

I normally use the G1 for everything but vld bullets, usually fairly accurate(hunting) drops out to 400 yards(range limit).

 

[HuntClub]

ok, here is what Sierra ballistics program came out with. ignore the path and wind columns as no gun info has been entered. Sierra also uses different BC's depending on velocity and also they have no G7 data for this bullet.

Thanks for sharing!

 

[Ned Ludd]

In his book, "Ballistic Performance if Rifle Bullets, 3rd Ed.", Bryan Litz provides a G7 BC of 0.220 as determined using Doppler radar for the [spire point] 150 Gameking.

However, if you can't find a G7 BC for a given bullet, it is pretty simple to fudge one from banded G1 BCs as follows: simply multiply the average of the banded G1 BC values by 0.51. It may not be exact, but will be more than close enough to work. Doing so with Sierra's values of 0.436 (2500+ fps), 0.430 (2500-2000 fps), and 0.410 (below 2000 fps) yields an average G1 BC of 0.425, and an estimated G7 BC of 0.217, very close to Bryan Litz' measured/determined G7 value of 0.220.

With regard to the predicted velocity issue, using G1 BC values should not cause the velocity to vary that much from that predicted using the G7 BC value. Although I much prefer to use G7 BCs whenever possible, the values obtained from a ballistic calculator such as JBM Ballistics will not differ so dramatically using either BC value. I would re-check all the inputs and settings to be certain something wasn't entered or set correctly in the program. As long as the correct valuers were used, simply changing the BC from G7 to G1 should not cause that dramatic a difference in predicted velocity at only 600 yd.

 

[David Milisock]

I don't have a 300 yard (nor a 200 yd) range i can shoot at.

Well that isn't convenient.

 

[CharlieNC]

You are most likely not using the program correctly, as you must tell it if you are using g1 or g2 and then enter the appropriate value. Even at 1000yd either will give essentially the same velocity.