We're into some interesting territory here.
"Is it better to step a G1 or G7 BC?" Truth is, if you're going to step the BC, it really doesn't matter which one you use, you will correct it with the steps. If you start with the BC that's a better match for the bullet you're using, then you should need fewer steps, and the required steps will be smaller, but neither is 'better' than the other if you're able to step any way you like.
Having said that, there are problems with stepping BC's, regardless if it's G1 or G7. All ballistics programs that have stepped BC's allow you to define the steps according to velocity. The problem is that a bullet's drag curve is a function of Mach number, not velocity. This becomes a problem when temperature changes (Mach number changes with temperature). In other words, if you go shooting in 59 degree weather and decide that you have to step down the BC at 1200 fps (for example), that step is applied at Mach 1.08. But if the temperature decreases to 20 degrees, the Mach step is applied at Mach 1.15 instead of 1.08 where you intended it to be (because the speed of sound changes from 1116 at 59 degrees to 1038 at 20 degrees).
This is a pretty minor problem, but it is a problem with the way steps are handled (even in Shooter).
Another issue is that in order to truly reflect the actual drag of a bullet thru transonic, it would take about 15 to 20 data points to really model the drag of a specific bullet, not the 3 to 5 data points that are typically used. This number of data points is nearly impossible to derive from shooting drops, which is the preferred method (and only method available) for most shooters.
There are other potential issues with using stepped BC's that I won't go into here.
So what to do?
I've long been an advocate of G7 BC's for modern long range bullets. I contend that for practical long range shooting (meaning ranges that precision is adequate to hit practical (game) sized targets) the single, averaged G7 BC is all anyone needs. Broz observation is that the single BC is good to ~1400 yards, and I think that's about as far as 99.9% of the population will ever attempt to shoot at anything they actually intend to hit.
Having said that, I recognize that there is room for improvement in trajectory predictions beyond 'practical' ranges, and for those cases, the single averaged G7 BC can be improved on. The new chapter in my second edition (chapter 11: Extended Long Range Shooting) eludes to this improvement, which is to model the drag of a bullet with a custom drag curve rather than reference it to a standard projectile. In other words, don't use BC's for the bullet at all, but rather each bullet has a unique drag profile based on it's shape which is represented with a table of cd vs Mach number points. This is the best way to eliminate error in trajectory predictions due to drag modeling error.
There is a new software app in development that will allow you to download custom drag curves for the bullets you're interested in. It will have a conventional library of BC's (just like shooter has), but for those ELR shooters who need to model drag near and beyond transonic, you'll be able to get the drag curves for those specific bullets. This app is intended for android, iPhone/iPod/iPad, blackberry, and windows mobile devices. I hesitate to talk about it now because the original 'due date' was June, and it's still not even in Beta testing yet. But it will be a reality some day.
My hope is that most 'normal' long range shooters continue to use the G7 BC's because they really are adequate for most users. But for that small fraction of people who shoot these extended ranges, there will be an option that is better for their purposes than a single BC.
BTW, when I shot 1 mile with the 300 grain Hybrids, my group was a little high using the single G7 BC, and so was the other guys group (different rifle, same bullet). When I compared my G7 based prediction to the custom drag, the custom drag would have had me even higher. This would seem to indicate the native G7 BC was closer than the custom drag, but I didn't get too worried about it because of other uncertainties in the test, most importantly range which was measured with GPS, not directly lazed. So there could have been some range error which is why I'm not concerned that the custom drag solution was further off.
Having read Broz accounts that his G7 based trajectories have him hitting a little LOW at ELR (and he's got a good laser rangefinder for those distances
) I'm encouraged that the custom drag solution actually does provide a better prediction when range is known with more certainty. As an example, at 1000 yards, the G7 and custom drag based predictions are within 0.1 MOA. At 1760 and 2300 yards, the custom drag predicted drop is 0.3 MOA more than the G7 based drop. This would bring Broz predictions into better alignment with his observations than the native G7 based drop.
There are other features of the new ballistics app which will make it more capable for ELR shooting, but I don't want to let the cat entirely out of the bag! Plus I have no idea when the programmers will finish so I don't want to stir up too much hype too early.
So to sum up; yes, ELR shooters do have a need for more sophisticated trajectory predictions than the customary BC's can provide. No, stepping BC's is not the answer, rather full custom drag curves for specific bullets is the right approach, and it's on it's way.
BTW, bullets and ballistics aren't the only links in the ELR chain. Don't forget about adjustable bases (which Shawn is working on) and reliable means for measuring range. I just sunk over $5,700 into a PLRF-15 OUCH! (thanks a lot BROZ!
). Effective ELR shooting is not cheap and there are few people with the know how to pull it off (but several of them are right here!)
Take care,
-Bryan
