Drag Function

[outerlimits]

in using various ballistic programs, i'm stumped about the drag (or g) function. there can be a significant difference in the result based on the "G" function. so, how do you determine that? some companys actually post that info, but others only the ballistic coefficient. any ideas?

[George]

Most mfgr. data is G1 drag modeled and is correct for a flat based projectile only. If the bullet in question is a boat tail profile, then the G5 drag coefficient is the correct one to use.

The upshot is that a boat tail bullet trajectory calculated with G1 drag modeling will be slightly optimistic. G5 lowers the BC a little bit comparatively.

Here is an example from the ballistics program I use which will actually convert on the fly for you (RSI Shooting Labs, Mac or PC):

.223 SMK 77

Published Mfgr G1 BC = 0.362

Same BC corrected for G5 drag model = 0.227

In the field, the G5 BC model has provided first shot hits at 500+ yards when using boat tail projectiles, whereas the G1 data has not given me similar results when used with boat tails.

[3GunF1Guy]

Here is a great article about BC.

http://www.shootingsoftware.com/coefficients.htm

I use the RSI software and put the bullet specs (length, weight, length of nose, base diameter), into the software and it always give me more accurate tables than anything else. What the table says is always what ends up on paper when shot.

Scott

[Guy Neill]

Generally, all factory bullet BC data is based on the G1 model, regardless of the actual bullet configuration.

Guy

[outerlimits]

i think what guy says is true. i've been playing with some nosler ballistic tips, which are boat tail and using the BC of .267, the trajectory mapping using G5 is not reality-it is much closer to G1 drag function, at least that's what the trip to the range shows...

[George]

Hmmmm!

I use Shooting Labs to convert the Mfg. G1 BC to G5 and then enter that result to get my drop tables. They play out right on the money for me. I have gotten elevation solutions that have yielded 1st round hits at out to 800 yards this way. The G1 based solutions were all way too optimistic and hit the dirt early ;-)

[ErikW]

I used Nosler's provided # for their 77, and calculated trajectories with G1. On paper, it put them exactly where the software said they'd be.