G7 > G1 ; resistance to BC change

So far I have spent zero time on the G1 vs G7 issue. Tell me though. If I check my drops using G1 out to my self imposed max range, does it then matter whether I ever switch over to utilizing G7 instead of G1? In other words is G7 an advantage only for longer-than-drop-verified distances? Or are there interpolation errors inherent in the use of G1 that don't exist when using G7?
 
YOUR particular bullet's BC is tied to IT'S particular drag at every velocity. Any G1 BC rating for your bullet is an approximate correlation to the G1 standard bullet drag model.
Throughout the velocity range of bullet flight, differences between your actual BCs and those calculated based on G1 cause prediction errors.
Most software utilizes an entered G1BC, throughout bullet travel, as though this is correct. Afterall, you entered it.
But if your bullet's drag doesn't match the G1 curve at every velocity downrange, then the local/incremental/software BC would likely be wrong. Well, right at some point in travel, and wrong everywhere else.
So if you adjust your BC for a match at a particular range, while that BC is actually erroneous about that point, you'll be most accurate ONLY at that specific range/BC match.

For instance, let's say you validate that .600 ICAO G1BC is tits at 1000yds, with an JLK boat tail bullet. Let's also say that the G1 curve does not accurately represent a drag curve for your bullet. What you would likely find with further testing at various ranges, is that the 1000yd validated BC produces fairy poor matches in POI to predicted, at both near ranges, and those beyond 1000. That results are best only at 1000yds(as validated).

A 190JLK with with a G1BC of .600, launched at 2845fps would be traveling 1500fps at a 1Kyd target. It's G1BC is:
.600 at the muzzle
.590 at 100yds
.578 at 500yds
.532 at 1000yds
This is an 11% deviation for that range.

Bryan Litz considered this error, built into today's mismatch between popular LR bullet drag and the G1 standard. He bracketed this error into a window, and adjusted Berger's BC to reduce (overall)deviations from predicted POI while using G1 BCs.
He chose BCs based on a more median velocity, rather than muzzle velocity.
This resulted in lower BCs, but also lower errors in the field.

BUT, the G7 drag standard matches the 190JLK bullet better:
.298 at the muzzle
.296 at 100yds
.290 at 500yds
.301 at 1000yds
This is a 4% deviation for that range. And Bryan has supported use of G7 as much as anyone.

Different methods are affected by this differently. Pejsa approach bypasses it, and is applied well in Loadbase. But nothing would be more accurate than incremental application of an actual matching drag curve.
Anyway, hopefully this helps with understanding.
 
So far I have spent zero time on the G1 vs G7 issue. Tell me though. If I check my drops using G1 out to my self imposed max range, does it then matter whether I ever switch over to utilizing G7 instead of G1? In other words is G7 an advantage only for longer-than-drop-verified distances? Or are there interpolation errors inherent in the use of G1 that don't exist when using G7?

When shooting at long range bullets will have a wide velocity range, If you've "tweeked" the drops at multiple velociy ranges (as Sierra publishes" you can't really say you're using the G1 model, only a series of interpolations. Doing that may give better results than swithing to the G7 model. There are several ways to fudge the numbers to get a better match for a set of measured data points. Doing that may or may not give better matches to points in between. The difference between the G1 and G7 models change their relative ratios slowly down to transsonic veloicities, typicallly below 1300 fps, then they can vary wildly because of the way the supersonic shokwaves form and detach between the two models. They return to being just a simple ratio below around 1000 fps, the what the ratio is depends on individual bullet shapes.

If you take sufficient data points and velocity intervals you'll get the same quality fix by adjusting either G1 or G7 coefificients. What your reallly doing as making another model to match your particlar bullet. The argument for using the G7 model instead of the G1 is that it's closer to start with for long ogive boattail bullets. Either model can be tweeked to give a better fit. Some programs like those from Pejsa don't use BC's at all. If you're going to curve fit to match actual drop measurements it's a simpler (and more logical) method. Tweeking either method if it gives correct drops willl allso give correct wind deflection.

It may not be obviious, but drop is directly locked to time of flight:
D=1/2 G *T^2 where:
D=drop, G= acceleration of gravity, and T=time of flight to that point on the trajectory. I believe Isaac Newton first published it.

Crosswind deflection is also locked to time of flight by:
Dw = Vw(Tf- X/Vm)
where Dw is deflection caused by wind, Vw is the crosswind velocity, Tf is the actual time of flight to that point on the trajectory, X is the distance to that point on the trajectory, and Vm is muzzle velocity. That is Ddion's equation.

Notice that BCs don't appear in either equation. They are only needed if you don't know the time of flight and need to calculate it from the bullet drag characteristics, atmosphere, and muzzle velocity. BC are just a number that is a multiplier to drag vs velocity table of a standard model of one shape of bullet. Those standard US Army models are numbered 1-7. They could have been any projectiles the US Army was using at the time but they made the firing tests and measurements to determine the drag curves for each one They weren't easy test to make with the instrmentation of the early 20th century. Today it's much easier to measure the drag function for each projectile design using millimeter wave radar to get a continous velocity, distance, and time profile.

If you have enough data points you can correct either G1 or G7 models to give an exact match to the actual drag function. But then what would be the point of using BC's at all instead of just using the actual drag function for for that particular pojectile design.
 
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If you take sufficient data points and velocity intervals you'll get the same quality fix by adjusting either G1 or G7 coefificients.

I take it you're talking about using multiple G1 BCs(per velocity) in software that allows it?
Like you said though, we might just tweek the actual drag curves. This would be more accurate than the few BC step changes provided by Sierra.

I can also picture seperate and unique drop/drift BCs in the future.
This, due to the Tlag drift influence, that is not a factor in drop.
 
I take it you're talking about using multiple G1 BCs(per velocity) in software that allows it?
Like you said though, we might just tweek the actual drag curves. This would be more accurate than the few BC step changes provided by Sierra.
I can also picture seperate and unique drop/drift BCs in the future.
This, due to the Tlag drift influence, that is not a factor in drop.


Yes to using sofware which handles muliple velocity segments. The one I use is the companion program to Quickload called Quicktarget. Whether that method is statisfactory depends on what you're trying to achieve. Sierra does provide multiple G1 BCs for many of their bullets. The results are ok except at transonic velocities.

If your ballistic program has external files for the G models or if you have source code and can re-comple the program with your own custom bullet models instead of using the standard G1-G7 models. One could working up thier own "G" function using shooting tests just as were done to generate the original G functions. Then that model would give the bullet used a BC of 1.0 at all velocities.

Gathering the data on the bullet is the difficult part. Multiple chronograh screens are the most direct method but they're unwiedy to set up and maintain. . A millimeter wave radar is the best but expensive. Here's an article showing what it can do and how to use it.

exterior ballistics

It also shows how Sierra determines the multiple G1 BCs they publish, it's limitations, and why it's better than a single value BC.
 
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