G1through G7 drag function..

[T-shooter]

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.

Litz tested a few of the tipped Match Kings:
http://bulletin.accurateshooter.com/2015/05/litz-field-tests-bcs-of-sierra-tipped-matchkings/

 

[shoot4fun]

Litz tested a few of the tipped Match Kings:
http://bulletin.accurateshooter.com/2015/05/litz-field-tests-bcs-of-sierra-tipped-matchkings/

Those Litz numbers are in JBM and what I trust.

 

[SheepDog]

The bullets are assigned a G1 or a G7 BC by the manufacturer or by an independent tester. Most American manufacturers use a G1 BC because the number is higher and high BCs sell bullets. The G7 BC number is always lower but it is governed by different drag slopes so even though the number is lower it computes the trajectory as accurately as the numbers are correct and the Drag group is properly identified.
You cannot enter a G1 BC and identify it as a G7 class bullet. You get garbage out because you put garbage in. The G number has to match how the BC is calculated.

 

[kvd]

One may calculate the ballistic coefficient with the following formula:

BC = (Weight of Bullet ÷ 7000) ÷ (Caliber)² X Form Factor.

The bullet weight is divided by 7000 to convert grains to pounds. The caliber is in inches (i.e 0.308 or 0.284). The units of ballistic coefficient are lbs/in². What we don't typically know is the form factor, which is a ratio of the bullet in questions drag curve to a "standard" bullets drag curve. According to Bryan Litz, most form factor ratios fall within the range of 0.45 to 0.60. The form factor is different for any given speed and thus the BC will be different at different speeds.

Of course, since we usually know everything except the Form Factor, we can rearrange the above equation and solve for Form Factor:

Form Factor = (Weight of Bullet ÷ 7000) ÷ BC X (cal)². This will be a rough estimation but may be useful.

Ken

 

[Ned Ludd]

The bullets are assigned a G1 or a G7 BC by the manufacturer or by an independent tester. Most American manufacturers use a G1 BC because the number is higher and high BCs sell bullets. The G7 BC number is always lower but it is governed by different drag slopes so even though the number is lower it computes the trajectory as accurately as the numbers are correct and the Drag group is properly identified.
You cannot enter a G1 BC and identify it as a G7 class bullet. You get garbage out because you put garbage in. The G number has to match how the BC is calculated.

If you go to Bryan Litz' book, "Applied Ballistics for Long-Range Shooting", you will find tables with determined average G1 and G7 BCs for hundreds of different bullets/calibers. Divide the given G7 value by the G1 value and see what you come up with. Do it for as many different bullets as you like. What you will find is that avg. G7/avg. G1 = ~0.51. It's because the drag functions/form factors of the G7 and G1 stds have that specific ratio. All of the test bullet dimensional properties are the same regardless of which standard shape you're using. So, in fact, they are relatively proportional to one another and can be calculated/estimated. In the manner in which these coefficients are typically used, it is perfectly reasonable and functional to do so.

BCs are generally given to three decimal places. You simply can't shoot that fine a difference. Further, BC will vary from Lot to Lot for a specific bullet, just as the bullet dimensional properties vary from Lot to Lot. We use these BC numbers as if they're written in stone but in reality, they're not. I often see elevation changes of half a minute or more during the course of a typical day at an F-Class match. The bullet BC didn't change during that time, the atmospheric conditions did. For that reason alone, it is safe to say that BC values are often not the limiting factor and a good estimate is more than sufficient [functionally] for most applications. If someone is into uber-long range shooting, I can understand that a good "estimate" might not be good enough, but for the majority of shooters, it works just fine.

 

[dcali]

By the way, since I didn't see anyone else answer this - the other functions (for example, G8) are not used with sporting ammunition, and ballistics calculators that include them are doing so just because they have the information. They are (were) used to model various types of projectiles that don't match up well with typical small arms ammunition used to shoot at ranges where ballistics calculators are useful. For us, G1 is good enough, and G7 is slightly better 99% of the time.

One thing people miss is that the BC is not a singularly definable number. What you are trying to do is overlay one curve on top of another when they don't match. Where do you want them to match up best? Mach 3? Mach 1? Do you want to minimize trajectory differences? How do you do that without knowing the actual drag curve? It's a judgement call that depends on the specific application. Fortunately, these judgements make only a small difference when you do it in a sane way, but it's a bit of an art as well as a science. Like Greg said, there's only so much accuracy we need in a BC.