Range Report G1 and G7 BCs , which is which and why?

[relentless1]

I have read Litz, s book on long rang shooting, but don, t get which bullet shape is which and for which BC to go by...I thought he summed it up by stating that the G7 is best to go by, yet others like Tib. Rex, seem to be using G1 in there bullistic charts for the sharp nosed long range bullets like sierra matchking, etc...?
Anyone shed some light on this please??

 

[jbailey]

How far are you planning to shoot and what will your bullet's velocity be at that point? G1 and G7 drag curves only start to differ materially at mach 1 - 1.2 and even then some of the error at faster velocities balances out the error at lower velocities. Chances are if you have to ask this question, you are unlikely to shoot the difference. With that said, use G7 when it is available (but make sure your ballistics program knows you are using the G7).

 

[Bob 964]

The G7 ballistic coefficient is used instead of the G1 because the G7 standard is a better match for the longer boattail type bullets most of us shoot, and should be more constant over a wider range of velocities, as compared to the G1.

 

[Rob01]

Should is the key word. In actual use the G1 works just fine. Been using it for years and continue to use it.

Another thread on the subject

http://www.snipershide.com/shooting/s4-sniper%92s-hide%AE-equipment/202240-g1-g7.html

 

[Lowlight]

They have been using G1 since the beginning, during much of that time G7 was also available however it never used, until now.

The amount of G1 Data available is deep, wide and very very accurate, why, because we have been using it for, oh 70 years or so.

There are programs out there, that will not even allow you to use G7, they get it done to beyond 2000m better than any "App" on the market, what do they use, G1 Only. (See Field Firing Solutions)

G7 as used by Bryan Litz is a very good average, and is supposed to give you a better representation of where the barrier of Supersonic Speed is... if you are not shooting beyond your bullet's capability than both work basically the same. Many find that "Apps" perform better with G7, because they are written to do so, however there are just as many people who are every ounce as successful with G1 in the same programs. For every G7 proponent you will find someone getting it done with G1.

Currently the biggest problem is, the amount of available data for G7 is highly limited as it is really only coming from one place. (although companies are starting to include G7 data) Also, people are finding out, since there are so many variables that contribute the effectiveness of the data, they have to manipulate the G7 number every bit as much as they do the G1. Today, programs like JBM will "band" your G1 data without doing any extra work so you have a really accurate representation of the curve, versus an "average" that was determined in one place, not several over 10s of years.

At the end of the day you can look for yourself very easily. Go to JBM online put in the data from the library with a bullet like the 175gr SMK and see what results are, then do the same thing with the (Litz) version of the same bullet from the JBM library. I think you will find the results are within .1 mils at 1000 yards, however the biggest difference will be what the software says for the MV at that point in the bullet's path. You'll have a difference there which only slightly changes the dope used. Now determining which one is right for you and your rifle is the trick... it still requires you to shoot it, and see, as not all of the variables are included in the computation, as well all of this stuff leaves out "YOU" the shooter, which has the greatest effect on accuracy. Some many find G7 to be spot on, others G1, still others will need an adjustment regardless of which one they use. I generally "know" the G1 data and use it with both FFS & CB1, but will used it when available in my Apps on the iPhone as those programs generally like G7 a bit better.

10 years from now, G7 will probably run the show, but until then, it is all a wash, same amount of work which ever one you choose.

 

[JB02]

Great answer! Thanks!

 

[relentless1]

Yes, thanks again Lowlight, you really know how to clear things up quick!

 

[damoncali]

If you look in the back of Bryan's book, you will see the actual drag data plotted, along withe the curves for the G1 and G7 drag functions. The data will fit one of those curves better than the other, which Bryan highlights for each round. But... the data is often sparse (just a few data points) and it's not uncommon for there to be a significant range of realistic velocities at which the two curves are basically on top of each other. Deviations tend to be at low and high velocities.

That said, real long range bullets look a lot more like the G7 projectile than the G1 projectile, so it's reasonable to assume that they will follow the G7 drag function better even when we don't have as much data as we'd like for every bullet. A hint that the drag function is not a good match is that there is a wide variance in the "banded" BC's given by Sierra, for example. If the drag function is a perfect fit, the BC will be constant regardless of velocity. If it changes a lot, it's an indication that the wrong drag function is being used.

In the end though, this only matters if you're into geeking out over this stuff. At normal ranges and velocities there is little practical difference. You just have to make sure you pair the BC with the drag function (that is, don't use a G1 bc with a G7 drag function) or you'll get garbage.

 

[Lowlight]

How is the "BC constant" when it is velocity based ? Are you saying the BC for the 175grSMK going 2800fps is the same as the same bullet going 2500fps ? Won't it make sense to adjust the curve based on the drop in speed rather than try and determine a one size fits all "average" for the entire flight ?

Banding to account for velocity changes works with G7 too and I believe Bryan is even promoting this now with G7.

If you shoot inside 1000 yards like most you can use an "Average" like you find with G7, but Banding works much better the farther out you go.

Pretty sure they have been aware of G7 since something like 1958, interesting it took us to post 2000 to start using it.

How a computer interprets the "curve" is a function of who writes it and with what program. Field Firing Solutions and Patagonia Ballistics do not use the same Point Mass model as all these other programs. Their Pesja Based Modeling is not Drag Curve dependent which is the trick. If you stop thinking in that fashion you'll get better results which is why their software works better regardless of the G Function used. It's unnecessary as it takes into account other factors and does well to bend the computer to the results like it should. (As we have real world data to include Doppler which both programs used to help the computer understand the flight.) When you tell the computer only follow "this model" you have issues, like people see everyday when stuff does not line up.

Pesja deviates a bit from what people believe is the end all be all to ballistics, and in my opinion, based on my experience using every piece of software out there, to include beyond 2000m, it shows when you compare them. Field Firing Solution which is G1 only works better than any Point Mass software on the market, so does Patagonia CB1, however he allows for the use of either G1 or G7. They have been tested side by side and alongside Doppler and their method is better.

Pointing to chart or graph and saying, "winner" because someone says they match is completely misleading. Real life does not demonstrate this.

 

[damoncali]

No, that's not exactly what I mean. Let me try to clarify.

BC's are not supposed to be velocity dependent. That is the entire point of having a BC. If you use a G1 or a G7 to approximate the drag function of an actual bullet, you will wind up with a velocity dependent BC because actual bullets do not perfectly mimic the behavior of the standard projectiles. That's a bug, not a feature. The goal is to minimize that variance, and that is why they have more than one drag function in the first place. Otherwise, we'd just use a spherical drag function (or whatever - take your pick) for everything and band the shit out of it. (Or, more sanely, skip the BC altogether and use a custom function). But that's a lot of work.

For the SMK 175, you'll see a greater dependence on velocity with a G1 than a G7, and therefore you'll have to fiddle with things like banding more. If you want the closest fit with the minimum fuss, G7 is the way to go. (Again, this is a pretty minor difference. We're way in the weeds here).

If you want perfect, it doesn't matter. You'll make adjustments to the drag function regardless of what you start with because real bullets aren't perfect fits to either function. What you are doing is creating a custom drag function; what you have is no longer a G1 or a G7, but something else. Banding a G1 or G7 or Gwhatever is just a mathematically convenient way to do that. Pejsa's work is mostly in solving this problem, from what I understand of it. (I'm not terribly familiar with it). Starting with a G1 and then modifying it because it was a bad choice to begin with is a bit kludgy, but it works.

I think what your'e saying is basically, "who cares what you start with because they're all a little bit off when you get way out there". No arguments there.

 

[Lowlight]

For the SMK 175, you'll see a greater dependence on velocity with a G1 than a G7, and therefore you'll have to fiddle with things like banding more. If you want the closest fit with the minimum fuss, G7 is the way to go. (Again, this is a pretty minor difference. We're way in the weeds here).

Well actually this is part of the problem... where did the G7 "average" come from and how many variations and factors went into getting that number, the minimum fuss is the part that doesn't match reality. Was the 175gr SMK shot and tested using a 16" bbl, 18", 20", 22", 24", 26", up to 32" or was it just tested with what they had on hand ? Cause reading the book it appears to be the later.

Using the 175gr SMK is a good example, because of the G1 history.

We all know Sierra Advertises .505 (@2800fps) however most are shooting it around 2650fps give or take a few fps. So over the years of experience with the 175gr SMK most have found .496 to be a great number, used a ton. Fast forward to today, and we see people are now using .475 in a lot of places, well where did this average come from, same place that created the G7 number I would say.

The lack of fuss is directly proportional to the amount of information available and the experience used to derive the information used. If suddenly 25 years of .496 has been reduced to .475 you have to ask why ? It worked well for a long, long time. So now we move to G7, that one number that is out there. .243... well i have used .239, 241, etc, it just depends on the rifle I am shooting it out of, so the level of Fuss is identical, except that, we know JBM for free will "band" your G1 BC automatically which helps give it a better result over a wider variety of rifles. Funny thing is, when you say"

I had to "fuss"with my G7 BC and change it from .243 to .239 all you hear back is, "ya but that is only X% less, look how close we are. Well .505 to .496 is close too, and I know enough now to shoot straight for that number. What difference is what percentage I am off, if I am Still Off ? Does it really matter, 2% or 4% not to me, maybe to a mathematician it does.

These computers have no clue about YOU the shooter, or your Rifle, except what you tell it. (if it even ask) So while someone might think they have 1-12 twist maybe they have a 1-11.25, or 1-11.8, or 1-12.5 they really don't know exactly and neither does the computer. (that whole 2% thing) We also know from experience that not all scopes are spot on. Some work great in a 4" box test, but will fall apart a bit when you dial past 30MOA. So that 1/4 click might be .27", .23" over the entire amount of travel. (as an example of this, the Gunsite XLR Course spends 2 days truing your system to include the software.) So there is another variable the computer is not aware of...

You can easily say, "well the science is, A + B = C, however life and reality is not that cut and dry. it's usually A+B / D = C which is where the fuss comes in.

The time in service is not there, the wide variety of experience to fine tune the number is not there, the basic information is not there with G7, so it requires exactly the same amount of "fuss" in most cases, while in other cases it requires, more because we now have to guess. With G1 it's less fuss because experience (read: time) has taught us, don't use .505, use .496, or .475... so they become go too numbers. Where .243 while a good average is clearly "one person's average" and not a huge cross section like .496., We some times have to hunt for the G7, as opposed to the G1, which is another fuss.

I am not saying is it wrong, not by any stretch, I am saying this is reality, and that reality is: Lack of Experience, which = Fewer Results with a wider variety of systems under it. I am all for arguing the science and I do my very best to study all the science I can, however my problem is, when the science doesn't work the same for "everyone" or the science was created in a vacuum, or the science was modeled and simulated more than it was shot. That is my issue... it's like we are the Beta Testers of G7, which is cool, I beta test it all the time, but the adamant statements made by some of the beta testers is blatantly misleading. Gravity is science and effects everyone equally, this is clearly not as we have a wide variety of results that don't line up. If every ballistician I spoke to agree I would be 2 thumbs up, but unfortunately they don't... some disagree on varying levels. So we continue to test, and verify, as I am willing to bet G7 will go down the exact same path as G1, which is a continuing modification of the data.

 

[damoncali]

I don't disagree with most of that. Garbage in garbage out.

By fussing, I meant banding, or using Pejsa-style corrections, not tweaking the BC's value (for the purposes of this discussion, I'm assuming we have access to reliable BCs for both drag functions. But there is an assumption in that - that there is a single BC. For the G1, there is clearly not. So we kludge it until it works. That's a wonky way to go about doing things, but it's gotten us pretty far.

The next step is to use a better drag function and abandon archaic calculation techniques that are really clever but rendered unnecessary by modern computers. The G7 is that function for most of what we shoot. It requires less wonky kludging to get right, which may sound trivial (how hard is banding anyhow? Not that hard.) But I argue it's not. If we are using a simple calculation method that allows us to isolate what matters from what doesn't, we will get to better answers faster and with less confusion. If you think explaining G1 vs G7 is tough, try explaining the pejsa methodk it's retardation coefficients, and whatnot.

Maybe it's wishful thinking to think it would be practical to use, but I have a mind to write a point mass solver that allows the user to edit the drag function directly. That makes for a clean solution without the compromises that plague Pejsa's method, limited only by the accuracy with which you can gage the bullet's true drag function. It's the best of all worlds. Simplicity for those who want it - using the best available data and standard drag functions, but with the ability to model literally any drag function you can imagine if your experience suggests that things are off somewhere.

 

[Lowlight]

I still don't see how can say with a straight face you don't have to change a G7 BC to match?

Maybe in a vacuum it is better, but people still adjust it.

As I noted earlier, I believe Bryan is moving towards banding G7 too, the real world almost requires it by default.

They are both equally, " Kludgey" when you admit the exact same amount of work is necessary in an equal number of cases to match up. Heck the software now is coming with "Truing" features, isn't that another cog in the system to line up the curve? Truing is not Kludgey ?

You idea of better is only on paper, nowhere else.

 

[damoncali]

Sorry if that wasn't clear - I never meant to say that the G7 is perfect. Only that a G7 requires less drastic modification than a G1. If you're going to try to modify standard curves, you should start with the best one available. The benefit to modifying drag functions rather than applying arbitrary non-physical mathematical tweaks is that the numbers you're tweaking actually mean something - you have something to go on when you modify them and they have a built in sanity check. More importantly, if you can make it work, you know why it worked and can learn something from that. Something that might make the next bullet easier to nail down. And yes, my idea is just better on paper (not that it's really my idea - it's just how a basic point mass solver works). I'm not saying this has any utility beyond obtaining greater understanding of long range bullet behavior and *might* be easier to do, although it's no less useful than the current methods of contorting standard functions out there.