Home Forums Hunting & Shooting Long Range Hunting Ballistic Coefficient discussion

I know that some bc measurements are more accurate than others. Looking at some of Bryan Litz's work and Sierra's numbers it would seem that bc changes as velocity changes.

example from for Lapua 155 Scenar from Applied Ballistics

fps / BCG1
1500 / .411
2000 / .458
2500 / .473
3000 / .493
Average/.459

Sierra Also lists bc's for different velocities as found on their website.

Questions:
-Do most manufacurers just advertise the bc for the higher velocity?

-Should use the average bc within the velocity range from muzzle to target when calculating or comparing drop/drift?

Numbers from a guy like Litz will usually be more accurate'er than the manufacturer.

Either way, numbers are just something to use to get you on paper. Only you can determine how great a bullet is or isn't.



Dave

^^^ BC's are, from manufacturer's, are a comparative value, this bullet higher/lower than that one.

With the LabRadar, which gives velocities at several user determined distances, you can calculate the actual BC for your elevation and environmental conditions. From actual bullet drops.

I use Litz's BC's to get in the ball park, they are actually pretty close.

He'll no doubt be calling you when he's ready to write his next book.

In general Doppler radar is the gold standard for external ballistics as prior to LabRadar the radar units would give you position vs. time for the bullet from muzzle until the transonic and beyond. I may be wrong but I think LabRadar units only give you information for about 200 yards.

To calculate the ballistics coefficient accurately you need to get the data closer to the transonic region when the bullet is dropping more more.

I have built a ballistics solver that matched up with the commonly held accurate solvers that would take radar drag curves or BC's as I was doing testing on a range that had a commercial doppler radar system. As a measure of how close Bryan's published BC's were I compared the results on 5 Lapua scenars, these were the only bullets that I had both radar drag curves and Bryan's BC's. The ballistic solutions with the BC and the gold standard the radar drag curves, were well under .25 MOA along the entire path. That is pretty good evidence that Bryan's BC's are very good for modeling trajectories.

I have no idea where Litz does his testing, at sea level or 6000 feet, or any varying environmental conditions, no idea if the twist rate of his barrel is optimized for the bullet or not.

The tools are available for the hobbyist today to model BC's and apply that to a drop chart. This is only done by actual field testing and known velocities.

I suppose this doesn't make any real difference to cowboy action shooting or any less scientific endeavors.

Exactly. So how is it that you are able to rationally comment on the relative accuracy of his findings?

The result of field testing concurs very close to his findings.

Field testing will reveal any anomalies that my conditions have as compared to his.

So, if I use a G7 of 0.324 for a 208gr A-max (this is a Litz BC) and plug all pertinent data into a ballistic program and find that my result is ,lets say, 2 moa different in drop than his then the 'effective BC' in my circumstance is different. If I know the difference then I can account for it.

This is but one of the items used to build an accurate drop chart.

Another example is a 200gr Accubond has an advertised G1 of 0.588 and a G7 of 0.273. The testing I have done indicates a G7 of 0.268. This is most likely due to a lower velocity, slower twist rate than normal (1:11), elevation and atmospheric conditions.

Commenting on relative accuracy of Litz's BC's was done through field testing.

You can do that too, the 'kicker' is you need accurate velocity readings, the rest is math and working the program.

As always, trust but verify.

If you don't know the conditions under which he conducted his tests, you have no idea how accurate his results are, and no basis upon which to comment on his accuracy.

That's correct , I do not know what conditions he tested under.

I do know that his BC's are more accurate than some manufacturers published figures.

I do know his figures are very close to my testing and ,as I said, use his figures to test reality in the field.

Now if Litz comes up with a BC of 0.273 and my field testing reveals 0.268, this does not mean my findings are better than his. It just means that I use the coefficient of 0.268 to build an accurate drop chart (for my conditions), that's all.

When I plug all the pertinent data into a Ballistic calculator (I have already verified the correct factor on my scope) and it tells me to come up 25moa to center at 1000 yards and time and again I need 26.5moa, this tells me the variable is the bullet's BC (I know my muzzle velocity and up to user defined distances), I then adjust the BC to reflect the actual come up or just ignore it and use 26.5 moa as the come up for a 1000 yard shot. This only works for one distance, better to plug in the new BC and test at varying distances to how ever far you want to hit.

So, anyone can comment on Litz's BC's. I use his as a default but also field test.

I am in no way challenging Litz's numbers as they are as close as they come.

I don't think anyone is attacking Litz' numbers. BC values will change from rifle to rifle. Individual rifling dimensions and the resulting engraving on the bullet will affect BC, so when a bullet manufacturer or third-party ballistician publish numbers, they are 100% applicable only so far as the testing/calculation techniques and test rifles used. Litz' testing uses the most advanced methodology that I'm aware of, which is why his numbers are the most universally applicable. But the actual BC value from your specific rifle may differ slightly.

The only trouble with determining the BC difference, is getting accurate and precise measurements on velocities and bullet drop at range. Often when a guy thinks his bullet's effective BC varies from what is published, what's really going on is that his chronograph has a 1% error resolution, or his rifle isn't accurate enough to determine drops with enough precision to identify any small difference in BC.

what's really going on is that his chronograph has a 1% error resolution


^^^ very true, optical chronographs will vary from one range trip to another as set up is critical and changing light conditions play with accuracy, along with the built in up to 1% error.

The Labradar claims 0.1% error and that's what I go with. Accurate velocity readings are the grail.

I wonder how a guy can tell if the 0.1% error rating is accurate or not. It's quite a new product, so user technique subtleties may not be apparent yet. The Shooting Chrony claims less than 0.5% error, but time and experience has shown that to not always be the case

A shooting chrony gives me an ES of ~30 fps over the last 25 years I've had it. Sometimes less sometimes more, but never settled down.

Some of these were very good loads according to the target, holding an MOA to 1000 yards.

Now my very good loads have ES's <10fps and I see it on the targets.

Accurate velocities made it possible to see the effect of changes I've done to lower ES, made the Quickload program work a lot better, which again shrunk groups.

Those 1/2 moa groups at a 100 yards that never stood up at long distance were ferreted out as to having unacceptable ES's that weren't seen on the target at 100.

So, is it 0.1%, I don't know but Quickload works as it should , Ballistic calculators work as they should, and if not you can find out why not.

Solving any equation that requires a velocity seems to work out now. The unit sits beside the bench and every shot is recorded which begins to build a good statistical record.

Back to the calculation of BC, correct velocity is the key for any calculations.

So, in short, I didn't just believe it, I worked it, as science is not faith based.

I'm glad it's all working out for you. Having said that, things usually work out for me pretty darn well using a Pro Chrono.

Precise velocities aren't the same as accurate velocities. Repeatable readings don't necessarily mean correct readings, which is the whole premise behind correctly calculating BC values. You'll still get incorrect BC values even is you have an ES of 0 over 50 shots, if all of those velocity readings are 100 fps too fast. The factors all working out as they should doesn't mean the velocity readings are correct, it just means that everything lined up.

Precise velocities aren't the same as accurate velocities

^^^ I don't what your after on that one.

The velocities that I get with the Labradar jive with known BC's when calculating a drop chart, due to the ease of recording every shot , a velocity average can be determined over a large sample size.

With this confidence in velocity other changes are much easier to see.

So if I have an ES of 20fps, over 100 shots I will average them , the velocity could be 2952fps. This is what I use to calculate BC's for external ballistics and Quickload for internal ballistics.

The lower the ES the better it will be for calculating either.

While not perfect it is much better than what has gone before.

If the math doesn't work then suspect inaccurate/precise velocity readings.

Its the foundation.

To actually address the questions asked...

Manufacturers advertise the highest BC number.

But some manufacturer provide 2 different types of BC numbers for a given bullet.

One is called g1 and the other is called g7... they are equivalent but different BC scales. A higher value is the sleeker bullet using either scale.

So for the most accurate results, when using the g1 scale, select g1 in your ballistics program, and plug in the multiple BC/velocity numbers into the program.

When using g7 numbers, after selecting g7 in the program, you can just use the single g7 BC number.

That way the calculated results should be similar.

Axtell, My hat's off to you! Your patience seems endless!

I still have some patience left, But now conserve what little is left for those who truly deserve it.

Y'all just need to buy some 165 grn core locks for a 300 ratherby. They are flat to 800yards, is the word. 🙄👌. And tasco 3x9 they rock too.

Smokes been upset sence this whole transgender stuff came up give him some slack.