Ballistic coefficients

[youp50]

Anyone know how they are calculated? Smokeaters modifications will obviously increase the BC of a bullet. Will that get a bullet to the near .300 that the BO has?

[joe21a]

Ratio of the sectional density of a bullet to its coefficient of form. This is the deff. To a dummy like me it is the ability of the bullet to overcome the resistance of the air during flight. the formula can be found at wikipedia.

[edge]

Normally they are calculated based on the shape and weight of the bullet. Some are shot over multiple chronographs and the rate of velocity decay is used to generate the BC. Rarely they are shot using Doppler radar.

If you have a bullet with a known BC and you make a bullet that is identical except for the weight, then the BC will be very nearly the same ratio as their weights.

For instance if you take a 250 XTP and a 300 XTP

The 250 has a listed BC of 0.147
divide 0.147 by 250 and multiply times 300 and you get 0.1764

The 300 XTP is listed at 0.180 BC

In general a longer heavier bullet of the same shape will be slightly better since certain portions of drag are not exactly proportional.

edge.

[rossman40]

The BC is calculated several ways as Edge posted. You can use a velocity method where you measure velocity at two different places (with doppler radar you can actually follow the bullet over most of it's flight path). There is a time method where you get one velocity reading and then time the bullet downrange. Then there is a static method where you work it out on paper. Then there is what Drag Coefficient was used in the calculation. The G1 drag coefficient is over 100 years old and does not give a good showing for newer designs like large radius secant ogive shapes, tangent ogive shapes and boattail bases. In the past bullet makers were famous for picking a BC method that gave the highest BC. The BC will also change depending on speed. The guys at Sierra will list sometimes 3 or more different BCs and speeds for their bullets where as other companies will not tell you the speed where the BC was calculated. For example the Sierra data for the .458 300gr HP list a BC of,

.120 @ 2400 fps and above
.145 between 2400 and 1900 fps
.185 between 1900 and 1400 fps
.210 between 1400 and 1150 fps
.230 @ 1150 fps and below

A Hornady .458 300gr HP, which is pretty close in build, is listed by Hornady of having a BC of .197 but they will not give you the specifics of how it was determined.

Many a shooter has been burned by believing the factory BC data, working the load at 100yds and then shooting at 600yds and finding out the calculated drop data is useless.

[Chris Champion]

Many a shooter has been burned by believing the factory BC data, working the load at 100yds and then shooting at 600yds and finding out the calculated drop data is useless.

Hopefully a shooter would find this out while punching holes in paper and not lobbing a bullet at an animal.

[craigf]

Jan 19, 2009 5:44:38 GMT -5 youp50 said:

Anyone know how they are calculated? Smokeaters modifications will obviously increase the BC of a bullet. Will that get a bullet to the near .300 that the BO has?

I don't think that it will change it that much. Here is what I do to Remingtons. They print the same at 100 and 200 yards, but the tipped group better. The difference in drop is probably seen at 300 yards. BOs print the same as the remingtons at 100 yards but drop less as 200.



The best way to figure out a BC is to get 2 chronographs that measure the same. What I mean is to put them up one directly behind the other and fire over them and they both get close to the same number. There can be big differences between 2 different chronos. Then put one at 100 yards and the other at the muzzle. The two numbers can give a accurate BC for that velocity range. This is more accurate than using one chrono and moving it because of the differences in velocities from shot to shot.

I'm sure that KerryB, Edge, or Richard can speak more about the how this works.

[youp50]

Craigf,

By going to the Barnes bullet sight and downloading the bullet chart, I find 2 Barnes Originals at the 300 grain weight. One listed as #457020 is a flat hollow point bullet with a BC of .227. The other is the 300 gr spitzer with a BC of .291. This is a 22% increase in the BC.

I am uncertain as the difference between Long radius secant and tangent ogives. I know that a gentle curving ogive will tend to increase the BC, due to less air turbulence in flight. Automobile manufacturers have been showing this with the newer style of bodies.

With different BC's for the same bullet based on Muzzle velocity I think perhaps the Wickepedia definition may be flawed. Ratio of sectional density to co-efficient of form. Sectional density is a constant for a bullet. Is co-efficient of form a variable?

I think I am more confused. Perhaps my normal state.

[craigf]

Personally, I think that the Barnes BCs are optomistic. The Remington HP BC is .213 and this works pretty good in the computer drop charts vs actual results. I find it hard to believe that the Barnes flat nose has such a higher BC.

[Chris Champion]

For the Barnes Original 300g Semi Spitzer I use a BC of about .250 for my 2400 fps load and that gets it pretty close in the drop chart that I use.

[youp50]

I just spent a good bit of time with the bullet guys from Sierra. Lot of good information there. I still need to digest it.

Some bullets have a lower BC as velocity increases and some have a higher BC. A bullet can go too fast and begin to yaw and the the BC goes way down. Plenty more where that came from.

I think Rossman is right on again. The manufacturers shop for the best BC. It may not be at the speeds commonly shot at. I think this is why Chris gets a true trajectory using a lower BC than advertised. I think Craig's observation runs to the same line. I am sure that nothing beats range time to develop your load's actual trajectory.

[rbinar]

8-)The ways used to calculate BC are varied and for some of us they are also wrong.

Now what I said above is not universal and if you have a better understanding of the process than I do I'd not want to start a "battle of wits" and end up un-armed. The value of BC has long been both an academic and commercial debate. It's sad but true that a company can print a number beside a bullet in a catalog and increase sales despite the meaning of the number.

In math the study of predicting details of moving objects is done in calculus. Remember that feeling when your mind went numb in a college classroom? Likely as not this subject was the one that convinced most of us not to be math majors.

The predictions we have now are based on models. A model is simply a guide based on measurement and probability. The problem has long been there is no perfect model (except Barbi Benton in the 70s). So results have varied over the years with different BCs plugged into different predicting (by math) platforms.

In an effort to provide better results devices for improving the models have been introduced into most platforms. This has been a boon for long distance shooters using bullets that are fairly aerodynamic. Now I have some problems with the nature of how the model is adjusted perhaps,but if it works it can't be criticized for results.

Where problems remain are with less aerodynamic bullets. It is entirely likely a ballistic program can't give an accurate assessment of a muzzle loading bullet with a given BC. That's because older models were overly generous with aerodynamic bullets and overly penal of less aerodynamic bullets. As time has passed the aerodynamic bullets have been adjusted by model but the others are still in 1972 (but I'm told not with Barbi Benton).

All of the above depends on at least a reasonable attempt to provide a number that fits with the model being used. As noted since BC sells most have a motive different from accurate predictability when stating a bullet's BC. This is often seen when similar bullets (same caliber, same SD, same shape) have different values of BC according to manufacturer. If you see this be sure you trust the lower number, if either.

So there is good news and bad news. Bad news is: good numbers are hard to get. Let's have a show of hands from those who dated Barbi Benton??? Good news is: For our purposes even bad numbers work better than expected.

[sw]

Hugh Heffner, but he went on to newer "models". Maybe she wasn't aerodynamic enough.

[DBinNY]

Here's Rifleman's (and Tar's) load with the 300 gr Barnes Originals. I know that lots of you also shoot this load. Here I just set the zero (Targ Dist) to 200 yards and played with the BC until it matched the observed drops as close as possible. This was done with Point Blank which there is a link to in tips and hints. Note that the BC that matched the observed drops closest was .215 and not the .291 that they advertise. This isn't perfect (1/2 inch high at 100 and 300) but it's close enough for government work. This is kind of a backward method of doing it but I'd bet dollars to doughnuts that you can do a good job of "filling in the blanks" when you have trajectory data at various ranges including the longest range you want to shoot. I wouldn't trust it beyond the longest known data point which is 300 yards in this case. I don't know (or really care) if this is the true BC or if it is skewed because the functions used in the software are from the '70s but it should give you reasonable trajectory data at all intermediate ranges for this load.

[wilmsmeyer]

When you look at a 300 gr sst and a 300 gr BO you see that the BO is slightly larger in diameter and profiled almost identical. The SST is advertised at .25 BC. The BO at .291. From a logical standpoint this means to me that the 300 SST is advertised too low (unlikely) and that the BO is advertised to high (likely) Looks like you kinda "proved" that out DB.

Nothing substitutes a paper test at all ranges. When you have those hard numbers at the ranges you shoot, the BC becomes irrelevant. One bullet will shoot flatter then another or it won't.

[youp50]

Barbie Benton was mind numbing enough for me. Calculus was deemed an out of body experience.

A couple of questions are raised in relation to this thread.

What does a person do if he is limited to a 100 yard range? Would using a ballistics program be ethical? How should one prepare for shots longer than 100 yards if limited to a 100 yard range?

Have you ever encountered a particular rifle or load that shoots flatter than should be possible?

[wilmsmeyer]

I think you can get by out to about 200 yds with a POI of a few inches high at 100 and going 2300-2400 fps. No program needed. Further then that I would want to paper test for sure.

Don't forget accuracy!!! If it isn't holding out at 200+ yds, a program won't tell you that...only shooting will!

As a last ditch resort you could have someone shoot for you at known speeds and use their data I guess. However, I'd want to do this myself...for my own peice of mind....and know that accuracy is there along with known drop.

JM2cents

[Chris Champion]

I agree with Wilms. Out to 200 yds at the speeds that we normally shoot the Savage is doable with the computer programs. I think much beyond that and the shooter owes it to the animal they will be huntin to punch paper at the distances he plans on shooting to verify not only the drops but especially the wind drift. Wind will play more havoc then drop with the bullets most of us shoot.