G1through G7 drag function..

[Rsadams]

Useing a shooters calculator it gives drag functions of g1 through g8. It's my first time useing it so what should I set a 168gr hpbt Sierra matchking in .30 caliber (.308 )? Is it G7?

Thanks in advance..
Shawn

 

[dmoran]

For actual use, which ever one coincides best with field results.
For ballistic study, I use G1 since most all bullets have G1BC specifications.
Just my 2-Cents
Donovan

 

[T-shooter]

G7 specs will be more accurate. You should use them whenever possible.

 

[T-shooter]

Here is a good article:

http://www.bergerbullets.com/a-better-ballistic-coefficient/

 

[Steve Blair]

G7 specs will be more accurate. You should use them whenever possible.

G7 will be more accurate for those bullets that approximate the model closely. This generally means long-nose, boat tail bullets. The G1 model is a relatively blunt nose, flat base bullet and bullets that resemble it are best approximated using G1. Below are the G1 and G7 models. Dimensions are in calibers. (Image credit: Kestrel Meters)

 

[Rsadams]

Thanks everyone for your replies...

 

[Rsadams]

Here is a good article:

http://www.bergerbullets.com/a-better-ballistic-coefficient/

I need to buy the Berger manual... They seem to be putting out some good data that alot of the other companies are not...

 

[T-shooter]

G7 will be more accurate for those bullets that approximate the model closely. This generally means long-nose, boat tail bullets. The G1 model is a relatively blunt nose, flat base bullet and bullets that resemble it are best approximated using G1. Below are the G1 and G7 models. Dimensions are in calibers. (Image credit: Kestrel Meters)

View attachment 1015763View attachment 1015764

True, you're right. Most of not all of the flat based bullets only have the G1 BC listed. And many of the boat-tail bullets only show the G1.

 

[Ray B]

In Handloader Magazine several years back they printed a formula for obtaining the ballistic coefficient. It was based on the actual velocities of bullets at various ranges, showing the deceleration of the bullet, which I understand was the method used by Sierra in determining the BCs of their bullets. How this relates to the form factor of the bullet I'm not certain.

 

[T-shooter]

Here is a bullet data base that may be useful:
http://www.shootforum.com/forum/bulletdb.html

And bullet lengths:
http://www.jbmballistics.com/ballistics/lengths/lengths.shtml#Sierra

 

[dmoran]

For illustration descriptions of bullet shapes, the G7 model can be a more accurate description for many bullets.
For ballistic solver/programs, coefficient values are all that is needed and used, and the bullet shape has no relevance. Sort to speak, ballistic solvers don't care what the bullet looks like or what shape it is, they just need to know the coefficient.
Donovan

 

[Ned Ludd]

IMO - the biggest practical difference between the two types of values is that the G7 values are less dependent on velocity change as the bullet slows down during flight. Most G1 values are banded within specific velocity ranges. Either standard BC value will work just fine, especially if you're using the appropriate velocity band G1 value(s) for the load and the approximate distance you're shooting. If you want to convert G1 to G7, just multiply by 0.51. To go from G7 to G1, divide by 0.51. This basically comes from the ratio of the form factors for the two standards (or the ratios of their drag coefficients), and will give you a good working estimate of one BC standard value if you already know the other.

 

[Mozella]

For illustration descriptions of bullet shapes, the G7 model can be a more accurate description for many bullets.
For ballistic solver/programs, coefficient values are all that is needed and used, and the bullet shape has no relevance. Sort to speak, ballistic solvers don't care what the bullet looks like or what shape it is, they just need to know the coefficient.
Donovan

What you say is correct and I'm sure you didn't mean to mislead anyone but someone new to long/mid range shooting might walk away with a misconception if they latch onto the phrase ".......... bullet shape has no relevance." So at the risk of picking nits, I'll say this.

Yes, a ballistic solver capable of accepting different drag functions doesn't care what the shape of your bullet is. If you put in a BC, it will give you an answer. But what most shooters really want is not just an answer, they want the correct answer.

I know you know this but the new guy needs to realize that basically, we shooters want to know the path of our bullet so that we can adjust our scope properly to get that first round hit under a variety of conditions. It's pretty obvious that distance, air density, MV, and bullet weight are key variables along with the force of gravity and look angle (up or down). Good programs include Coriolis effect and other smaller factors too. This stuff is pretty easy to understand/calculate.

What's harder is figuring out how the bullet slows down on it's way to the target and that depends on the bullet's drag which is definitely not linear and is quite tricky to describe accurately. The best we ordinary shooters have to work with at the moment are the published or tested BC's associated with whatever bullet we're using. Typically all our bullets have a published G1 BC and most long-range target bullets also have a published G7 BC. Reading the links mentioned in this thread will explain why mid/long range shooters using low-drag, boat-tailed bullets should rely on the G7 information.

As you say, there is no need to input bullet shape to a ballistic app but bullet shape is not irrelevant. Bullet shape is important and that's why shape is included in the drag model, G7 for most of us who shoot at mid/range targets.

Bottom line: G7 data is appropriate for F-Class shooters, to use one example, because our bullets look something like the G7 "standard" bullet. Even though a ballistic app doesn't "care" what the bullet looks like, the shooter should care and pick the appropriate drag model which has the bullet shape built in.

 

[kvd]

Useing a shooters calculator it gives drag functions of g1 through g8. It's my first time useing it so what should I set a 168gr hpbt Sierra matchking in .30 caliber (.308 )? Is it G7?

Thanks in advance..
Shawn

Shawn,

Use G7 if it is available. Otherwise use the G1 value given for a particular bullet.

Ken

 

[dmoran]

@Mozella -
Cool with your added details, but like you, feel someone new to long/mid range shooting might walk away with a misconception if they latch onto the phrase "Bottom line: G7 data is appropriate for F-Class shooters......"
So at the risk of picking nits, I'll say this: G7 is not the only appropriate data for F-Class shooters.
Besides G7 as you state, G1, 4DOF, and Doppler based ballistics all will work ideally as well.
Good input !.!.!
Donovan

 

[CharlieNC]

Unless you shoot ELR it won't matter. Studying a wide range of bullet calibers there is a perfect correlation between G1 and G7 values, meaning the characterization of the bullets is identical. Running calculations for both in different ambient conditions, the resulting ballistics have been virtually the same. The G1 and G7 drag curves differ significantly as the velocity decreases at the speed of sound, which is where the difference becomes significant. I still run both on my phone, and the most important aspect to achieving accurate ballistics is keeping good dope for calibrating the app properly.

 

[dmoran]

^^^^ very true !.!.!

 

[Rsadams]

Thanks everyone, I was trying to get an idea of bullet drop on the 168 matchking for the .308 and I am new to the caliber.. It makes a lot of difference switching from g1 to g7 in that department becouse it looks like its carrying more fps further...

Right now the furthest I can test is 300 yards and am still in load development at 100 but am stuck becouse of the 100+ degree heat for the next month or two. I have just stopped becouse I won't be shooting in this kind of heat.. So with that said I will probably be stuck at shooting under 500 yards. Ranges seem to be shrinking but with all the guys that show up with .338 lapuas these days maybe there will be a need to expand ranges in the future..

I just don't think alot of those guys will stick with it.. Honestly I think they think you can just go buy a nice rifle and a box of factory ammo and shoot 1000 yards easy..lol That plus none of them reload as they say. (YET).. I hope they do stick with it I try and help anyone that ask me to further the sport..

I am just a guy that was raised shooting , been doing it for 40 years and have always loved it since my grandfather showed me how as a little boy.. It has brought me so much I try to pass it on to the younger people out there so it doesn't die off.. I just see alot of faces that don't come back when they realize its both expensive and not easy as TV makes it seem..

 

[T-shooter]

You need to know what the velocity at the muzzle is. I put my chrono out 2 yards and then calculate the velocity difference. The .308 rounds slow maybe 5-7 fps. A program that you can use to calculate it. Change the velocity until it matches the chrono at 2 yards.
http://www.modernballistics.com/download.htm

I ran my specs on 3 different free ballistics programs for a comparison. Long Range Hunting (G7), Hornady 4DOF, and Applied Ballistics. Actually pretty close numbers between all 3 using G7 specs and calculating for spin drift and Coriolis at 1,000 yards:

LRH Drop= 395.10" Total windage (no wind)= 11.50" Velocity= 1,322 fps

Hornady Drop= 391.14" Total windage (no wind)= 9.92" Velocity= 1,324 fps

A/B Drop= 393.63" Total windage (no wind)= 10.43 Velocity= 1,325 fps

 

[shoot4fun]

I use G7 on G7 shaped bullets and G1 on G1 shaped bullets. Sierra doesn't seem to provide any G7 data for even the newest SMKS (pointed or tipped) so I get those values from JBM mostly. It would be nice to see, now that Sierra has recently done so much to upgrade their bullets, factory provided G7 for their bullets that fit the model.
I use very few of the flat base type bullets that fit G1 these days and that is mostly for short range plinking/hunting.