G1 vs G5 calculating BC?

[RockyMtnMT]

Hey guys,

Can someone explain what the different drag coefficients (I don't know if coefficient is the right word) mean to the b.c. value of a bullet. I have played w/ it in a ballistics program and it makes a huge difference in the drop chart. Some manufactures say they use G1, others I have seen say G5, and most do not even say.

Thanks, Steve

 

[crusier_32]

I think you should use the G1 for flat based bullets and the G5 model for vld's. Here is a site a quick google search brought up.

exterior ballistics

Hope that helps.

Will

 

[mikebob]

I too have played around with that. I was using the G7 (VLD) drag function. To make the charts come out right i had to drop the bc from 517 to something like 254. I got it to actually match closer than using the G1 function. My charts match at zero and 1000 but have been little high at mid range, using the G7 function corrected the mid range charts. Dont really know what it changes but i do know i had to drop the bc way down, like half to make it work. I too would like to know about that i think i will search around and see what i can find.

 

[Michael Eichele]

The purpose of a ballistic calculator is to help you get on or near paper at long range and once you find your real world data such as drag model and BC this allows you to accurately and reliably predict where your bullets will impact in different atmospheric conditions. Real world testing is needed to get the best drag model and BC.

In my experiance, it depends on the bullet AND the rifle it was fired from. For example, the 155 SCENAR in theory should be a G5 or even G7. In my "real world" tests with one rifle in particular, a G1 model of .499 matched the RSI program and TRA near perfect. The 178 AMAX in the same rifle was best used with G5 and .318. These numbers were not figured by programs or guesses but actual tests.

With the differing velocities we shoot bullets, barrel quality and twist rates it can be near impossible to predict an accurate drag model and BC and IS impossible to accurately predict one number for everybody to use with any reliablity.

My advice is start with G1 and the factory number and go out in the field and start there. You may be right on or not. You in MOST cases will get on the target somewhere. Once there, you can work into the bull's eye and document the difference. Then go home and work the numbers on your program. You will find a "G" function and decimal number to coincide with your tests. This is what you will use for future ballistic calculations.

Also get used to the fact that higher G numbers ussually have lower BC numbers. This doesnt meen the bullets will fall off the table. In my previous example between the 178 and the 155 you will notice the HUGE difference in the decimal number yet when I used the G1 model with the 178 the number was somewhere around .5 (I dont remember exactly) and that got me from my 300 yard zero to 1000 yards. The problem was the ranges in between my zero and 1K and the impacts did not match the calculated drops. When changed to G5 and .318 everything matched with near perfection. I think the worst number on the calculated drop chart was less than 2" off the mark from the real world drops at 700 yards.

 

[mikebob]

Meichele that is the same thing i found about mid range. I still use G1 function and shooting to figure actual charts. If I read the lit. right i found on internet the different G functions actually change the bc spread down range according to velocity loss. So instead of having a high original bc like factory publishes it is an average bc of total flight of bullet, thats why its lower. It got way over my head but I think know after some reading you would be alot more accurate if after initial charts made switch to actual G function of bullet your shooting especially if you change conditions extremely. For me where I shoot in conditions that do not change over 3000 ft altitude I dont think it make much differ but for guys shooting extreme differ conditions it might give better readings. Hell I dont know, I do know i am going to have to practice my butt off to be able to shoot the differ. in the few inches it made in mid range for it to matter.

I was just on JBM website reading about drag functions and found there is a calculator on there that figures your different drag function model bc's. I tried it with what i found in my rifle going from G1 to G7. It gave a bc .001 higher than what i came up with, you cant get much better than that with a program.

 

[BryanLitz]

Here is a link to an article I wrote that talks about referencing BC to alternate drag standards, G7 in particular:

http://bryanlitz.bravehost.com/articles/Berger 155 grain VLD.pdf

You may find it interesting.

The general shooting world may not be ready for bullet makers to start publicly advertising BC's referenced to other standards just yet, but I think it's close. In the mean time, I will be happy to provide G7 BC's for Berger bullets on request ([email protected]).

It's good to see discussions about technical stuff like this.

-Bryan

 

[mikebob]

Thanks Bryan,

That is exactly what i was looking for. The other articles i found seemed to say the same thing but did it in a foreign language to me. That article is very well written and a lot easier to understand for this redneck okie. I think i will start using the G7 drag function from now on to figure my bc and charts. I will be sending you an email for G7 bc on my berger bullets.

Mike

 

[Jeff In TX]

Lots of ballistic programs us G1 drag model as the basis for their calculations. The problem is the G1 drag model was developed over 100 years ago is a based on a 1 pound 1 inch round nose projectile. Modern bullets don't fit too well into this model built for a 1 pound 1" round nose projectile.

However, bullet manufactures use the G1 BC to advertise their bullets BC's. This is because G1 drag model produces higher BC's numbers and higher BC's sell more bullets. Bullet manufactures calculate the G1 BC's by a computer and most times out 10 won't actually match the true BC of bullet out of your gun. So if you're using the advertised G1 BC you're probably already starting off with incorrect input data.

Modern bullets such as boat tails fit the G5 coefficient drag model and VLD's fit the G7 coefficient drag models. However the G5 and G7 coefficient drag models produce lower BC's but there ballistic calculations are very accurate, much more than the standard G1 drag model. Lower BC numbers won't sell as many bullets...marketing marketing marketing!

To correct and compensate for the short comings of the G1 drag model, ballistic programmers massage their G1 ballistic calculations. One way to do this is by using multiple BC's. However it's difficult to accurately predict or measure all the de-accelerating BC's. So a lot of time you're guessing and that's not a very good way to get accurate results.

In the past these G1 massaged calculations were mediocre at best. Today's ballistic programs have much better mathematical calculations and have good to very accurate calculations. However, the best method is calculate your bullets exact BC and match it the correct coefficient drag model. Or, at least convert the advertised G1 BC to the corrected G5, 6 or 7 depending on the bullet type and use the correct coefficient drag model on your ballistic program.

Hope it helps

 

[RockyMtnMT]

Thanks for all the input guys. I like this tech stuff as well. This explains why the bc's for the GS bullets doesn't look good. They claim that theirs are measured on the G5 model. Run their #s on the ballistics program and the drops are very impressive. I will have to spend some more time playing w/ them.

Thanks again, Steve