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Can anyone explain to this slow cowboy the difference between G1 and G7 ballistic coefficients?
Simplify, as I'm not tpo bright

They refer to different bullet "models." The G1 model is a flat-based, tangential-ogive spitzer. The G7 has a long boattail and a more tapered ogive.

Thanks, John - I'm betting there exist other models?

As I recall... There are many standard "model" bullets. Charts exist showing how much drag each one suffers at any speed. You pick a model bullet that looks about like yours and assume the drag curve has the same shape. Measure your bullet's speed at some distance and compare this to the muzzle velocity. After adjusting for your bullet's weight (heavier bullets slow down proportionally less), you can figure out a fudge factor to apply to the drag curve to match your measured deceleration. G1 was the original shape used; it approximated an 1800s artillery projectile. G7 is closer to modern bullets in shape. The G1 and G7 drag curves are rather different, as you might expect. Coming up with a fudge factor to make a boat tail spitzer calculate like a old howitzer shell will, of course, not be perfectly accurate, but it works surprisingly well.

Mark,

Yeah, there are, but with the present ability to actually measure BC with Doppler radar, there's really no need to rely on "fixed" models. There's plenty of Doppler-verified info on various bullets available these days.

Explaining the problem in a different way might be useful.

At subsonic speeds, one can calculate a "drag coefficient." With this one number, you can predict trajectory knowing only the mass and frontal area of the projectile.

As one approaches the speed of sound, the drag coefficient rises and peaks around the speed of sound. As the speed increases, the drag coefficient drops. A projectile's shape, especially near the nose, determines the shape of a plot of drag coefficient versus speed, and it varies quite a bit. A single number will not accurately describe drag over the supersonic range of speeds, therefore we refer to standard projectile shapes for which the drag curve is known and assume our bullet's drag curve is proportional to that. This is only an estimation, but it works. The accuracy of the estimation is improved if you pick a reference projectile whose drag curve is actually similar to that of your bullet. However, to compare bullets of markedly different shapes, we adjust the same drag curve (G1) to all of them. This is okay for relative comparisons, but if you want to calculate drop at 400 yd down to some tiny fraction of an inch, you should use a better matched drag curve, such as G7.

Estimating the fudge factor for the G1 drag curve using chronograph data is not reliable. The atmosphere is quite variable and throws off the subtle calculations. As MD points out, today radar measurements can accurately read velocity changes over a small distance and remove much of this variation. You get a more accurate BC number, but any BC number references a particular drag curve.

Today's computing power allows one to estimate the true drag curve of a bullet starting with its shape. Hornady has a trajectory program, but I think it uses stored drag curve measurements for each Hornady bullet instead of doing calculations to form the curve. Regardless, this will be more accurate than using the drag curve of a reference bullet. However, out to 100 or 200 yd, it will not be a big difference; your hold will be a greater variable.

A little work with google brings this page which shows the great difference in drag curves. They all have loosely the same shape, which is why a single BC number can give respectable predictions, but the details are enough different to affect calculations at long distances. Note that at low subsonic, the drag coefficient is constant.

It's kind of funny how they tout the accuracy of such modeling. It's not as if we take a weather measuring station into the hunting fields to let us compute true air density.

2525,

How are you defining "a small distance" for Doppler radar measurements?

The Doppler gives instantaneous velocity and range, and it can thus provide a (nearly) continuous plot of velocity. How small depends on the rate and accuracy of the radar's sampling.

Exactly. That's why I asked what you considered a small distance.

I knew what you were getting at, MD. In my earlier post I wanted to avoid bringing in radar accuracy and sampling rate, so I just called it "small distance." To get a useful deceleration measurement with the radar, the distance must be enough that the variation in the radar's Doppler measurement isn't too large a percentage of the actual velocity drop.

To tell the truth, until after I made my first post and did some looking with google, I didn't realize, as you do, that today's labs are generating true drag curves for each bullet: Neat.

Not only is improved instrumentation able to derive the exact drag curve for each bullet tested, but testing reveals that the drag function is related to more than just the bullet. Shooters take published load velocity numbers with a grain of salt; we accept that velocity will vary from gun to gun. We find that the same is true for BCs and drag functions. Perhaps the best summary is available on Hornady's web site where they discuss their 4-degree-of-freedom model and radar data. Bullets from the same lot behave differently when fired from different guns. We are still learning.

I thought I'd provide a link to the Hornady discussion which KenOehler mentioned, and in looking for its location, I see the software runs on their web site, and includes bullets from other makers (but not the 220 RN in use in my .30-40). On the page is a link to the calculator page, as well as a link to a .pdf giving more information on their computations. I'll have to play with their calculator and compare it to BC derived trajectories using their published numbers, in the JBM calculators. Time to geek out!

That’s very interesting !! I don’t necessarily like it. It >> muddy’s << up my mud hole. Apparently all this ballistic stuff is not as simple as some thot. ---->---> [/b]HOW do you spell muddies/muddy's/mu..?? I don't think I've ever spelled that.[b]

Thank you Dr O. Always glad for your insight & input.
BTW. My 33 still works perfectly. I’m glad, you might not be.

Jerry

Thanks 2525 for your direction to this info. Appreciated.

Jerry

For giggles and grins I compared Hornady's "4 DOF" computations to the older, BC based methods. I used the JBM calculator and a simpler JavaScript based one. The JBM supports G1 and G7 and the other only G1.

I used a 200 yd zero and 10 mph 90 deg crosswind starting at 2600 fps and calculated the drift and fps at 200 and the MRT at 100.

Here's the numbers for the 220 ELD-X, for which they publish a G1 of .650 and G7 of .325:



and here's a less fancy bullet, the 178 BTHP of .530 G1 (for which they don't have a G7 BC):



I'm amused by how good the G1 estimates compare to that from the orgasmatron, at least at practical ranges. I imagine Hornady fit the G1 BC to the range of speeds used here (starting 2600 fps). Maybe I'll go try some trans-sonic runs. The BTHP G1 BC may be an older estimation.

The "simple" JavaScript based calculator likely uses a cruder numerical integrator and drag table than used by JBM (where the calculations are done on their server, not in your browser). It looks as if the coding for the wind drift computation may be slightly off in the simple version.