J,
Good questions.
It's true there are several other standard projectile shapes other than G1 and G7. It is possible to define a BC for any bullet in relation to any of those standards. Ideally, from a technical point of view, you should choose the standard that exhibits the least variation. For long range bullets, G7 is much better than G1, but in some cases, G5 can be a slightly better match than G7.
To put it in perspective, using G7 referenced BCs will usually erase 90% of the velocity dependence of a G1 referenced BC. In some cases, a particular bullet may have an additional 5% improvement if referenced to some other standard (like G2, G5, etc).
Although it may
technically be a better solution to use G5 in some cases, I offer the following arguments against that practice:
1. If you start using many multiple standards to define BC's, you loose the ability to compare bullets based on BC. For example, if 'bullet A' exhibits a slightly better match to the G7 model and it has a G7 BC of .237, and 'bullet B' exhibits a slightly better match to the G5 model and has a G5 BC of .342, you can't say which is better based on BC (not without converting, which is a lengthy and complicated process to do right). My point is that it's better, from a standardization point of view to adopt one standard rather than using multiple different standards. You're still eliminating
most of the velocity dependence by using the G7 standard.
2. The experimental data isn't always good enough to determine which match is better. My experimental data is very good at generating a reliable
average BC (regardless of the standard used). However, the variation in BC, which is determined from the exact shape of the drag curve, is much harder to nail. So if you have a bullet with 0.006 variation in G7 BC and the same bullet has 0.010 variation in G5 BC, can you say for certain that the G7 is a better match? Not with my data. My data is good enough to resolve that a G7 (or G5) BC has much less variation than a G1 BC because the difference is so huge. However, the difference in the shapes of the drag curves between G7 and G5 is so subtle, and honestly, my data isn't good enough to say for sure which is a better match for each bullet. Naturally, since the difference is small, it has a minor effect on the accuracy of the trajectory you'll calculate.
You may notice that the above 2 considerations both compromise on what's
technically correct. However, they both do so in order to make a solution possible that's better and more useful than what we already have.
There's nothing technically wrong with using a G5 BC for a bullet that matches that standard better, as long as you have reliable data that indicates that the projectile actually does match that standard better AND you understand that you can't compare a G5 BC with a G7 BC.
My prescribed advice of using G7 BC's for long range bullets is a balance of many considerations including what's technically right, and what's a
practical solution that the majority of shooters will be able to apply. A solution that's perfect in it's technical completeness will probably not get off the ground because it's overly complicated and has too many 'gotchas'.
Regarding ballistic tips in particular, it wouldn't surprise me if they match G5 better than G7 for the following reasons:
1. Tangent ogive vs secant (the G5 model is tangent, G7 is secant)
2. Short steep boat tail. (the G5 model has a short BT, but it's not as steep).
Having said that, I would expect that you will erase most of the velocity dependence by using a G7 compared to G1, even if G5 is slightly better. It probably amounts to a fraction of an inch at 1000 yards, an inconsequential amount.
Here's an image of the G5 standard projectile:
The dimensions in the figure are in 'calibers'.
Really good questions, I hope I've helped to clear it up.
-Bryan
