Let's argue about BC's

[groper]

I was just wondering the equation of how they calculated wind drift. I agree it is impossible to judge the wind. I was just speaking in general terms and not a specific incident. Does the equation take into effect bullets don't drift in the wind according to their frontal bc. I have fired two or more rifles simultaneously side by side at my range so they experience as close to the same wind as possible. Drifts are different with same bc from heavy large calibers to lighter smaller calibers. Has anyone else ever done this and noticed the effect.

At a 1000+ yards I have seen significant differences and not just small gains in a strong wind. I don't want to hijack Paul's thread so I will stop. I was just wondering.

Ive never tried it, but im not surprised by your findings...

See the BC is made up of 2 Main components, one is its energy and is therefore governed by its mass.

The other is its aerodynamic drag, which is totally unrelated to its mass obviously.

So for a light bullet, vs a heavy bullet, both with the same BC, one bullet has less aerodynamic drag component, the other has more drag but makes up for this with more mass to arrive at the same BC.

So when we are considering wind drift, the inertia of the bullet, is what the wind is fighting in order to accelerate the bullet sideways, so i would say the aerodynamic drag properties are not as important as its mass properties. ... therefore i would expect its main governing factor to be the bullet mass and.... side profile area- which would govern the magnitude of the wind force for a given wind velocity... The frontal aerodynamic drag i would expect to have little/no effect in this regard, and i would expect it to explain the difference in wind drift from heavy/light bullets with same BC - although i could just be talking out my ***

 

[MontanaRifleman]

Ive never tried it, but im not surprised by your findings...

See the BC is made up of 2 Main components, one is its energy and is therefore governed by its mass.

The other is its aerodynamic drag, which is totally unrelated to its mass obviously.

So for a light bullet, vs a heavy bullet, both with the same BC, one bullet has less aerodynamic drag component, the other has more drag but makes up for this with more mass to arrive at the same BC.

So when we are considering wind drift, the inertia of the bullet, is what the wind is fighting in order to accelerate the bullet sideways, so i would say the aerodynamic drag properties are not as important as its mass properties. ... therefore i would expect its main governing factor to be the bullet mass and.... side profile area- which would govern the magnitude of the wind force for a given wind velocity... The frontal aerodynamic drag i would expect to have little/no effect in this regard, and i would expect it to explain the difference in wind drift from heavy/light bullets with same BC - although i could just be talking out my ***

Not sure where you're talking out of but I used this quick and easy (but accurate) online balistic calc, which I've done before, and it tells me that only two things matter when it comes to Windage, Drop and ToF... and they are velocity and BC. You can change any other parameter, but as long as the BC and velocity remain the same, the Windage, Drop and ToF remain the same.

External Ballistics Calculator

EDIT: You can change any other bullet parameter such as cal or weight...

A 50 gr .22 cal bullet with a BC of .5 shot @ 3000 fps will drift, drop and get to the target at the same as a 300 gr .338 cal bullet with a BC of .5

 

[ZEEK]

I was looking beyond the obvious things like bc's changing with velocity and atmospheric conditions which do affect the bullet but not as great as the discrepancies Paul and I are getting. JWP416 was in areospace missile work like me I think, maybe he has some ideas.

Here is another one that involves the most difficult part of long range shooting which is reading the wind. Wind effects are the primary reason I spend so much time testing a bullet when I change from one to another. BC's are calculated for frontal wind resistance and that bc is applied to wind from other angles (wind drift) which is inaccurate other than for the time of flight portion of the equation. How are the side profiles of the projectile and mass considered with the wind vectoring in at different angles and different speeds. You can guestimate the wind vector fairly close with new stuff out there but the frontal bc of the bullet is still used and not the actual realistic side profile and mass of the bullet. That is why it is so hard. Wind effects on the side profile are also different with all bullets and that may or may not correlate to the frontal BC. So how exactly is wind drift calculated by most ballistics programs. One of my degrees is in applied mathematics if anyone knows where I can get the formulas to go over them.

I have found that the wind portion of the ballistics tables are off the most with different bullets and you just have to know your particular bullet and have experience with it. I have found that for the highest bc bullets the tables are closer which makes sense but for some of the mid range bc bullets they can be off quite a ways. It seems to me to be right on with accuracy you would need a side profile wind variable to plug into the equation for each bullet just like the frontal bc that predicts how well the bullet resists atmospheric conditons from the front. With aircraft it is easy since we have rudders to vector the aircraft to a point in space. But with bullets we must predict the wind effect and change our sight point to where we think the bullet is going.

Experts care to shed some light on this. Or did I take one of my missiles off in space somewhere.

I think you will find the "math" you are looking for here:
Accurate Versus Low Wind Drift Cartridges

 

[Augustus]

The drops at 1000 yds of a .9BC bullet with a MV of 3250 and a bore ht. of 1.5 will be identical to a .8 BC bullet with a MV of 3250 and a bore ht. of 2.38. I am simply amazed that errors such as this are being made by bullet makers. Bryan Litz came out with an advertised BC of .9+ on the .338 300 gr. hybrid and as soon as it got into the hands of shooters it became painfully obvious that the .9 was wishful thinking. The HATS was hyped up to have BCs around 1.1, what a joke.

It does not take a rocket scientist to take a projectile and do velocity and drop test that can be counted on to be replicated by the shooting public providing reasonable care is taken to insure all the inputs are correct.

Paul, I do not mean to be rude and I appreciate your efforts in finding the input error and reporting it. I have always doubted the BC #S being floated around on some of the custom bullets. Always remember, when testing bullets if the BC results seem WAY to high, they are probably WAY to high and one should start checking things like MV, BORE HT, ZERO, UPDRAFTS, WEATHER INPUTS etc. anyway thanks for the transparency. That is a commodity in very short supply these days.

 

[Long Time Long Ranger]

Paul, thanks for advising us. Your bullets are still top notch and a true .74 bc is awesome for a 7mm bullet.

Groper, I understand what you are saying and where you are coming from. That makes sense, I just wonder if any of the bullets resistance to side forces such as wind is in the equation.

Montanarifleman, That is my point that if you take the same bc and velocity, etc it will spit out the same results. However I have found discrepancies in that.

Thanks Zeke, I will look at the formulas and see how the wind variable is calculated. My guess is it is straight from the bc because like stated above any bullet with the same bc gets the same results. I know that wind affects different mass and shape objects in different ways. So I will look for the variable for this. In areospace work both the frontal and side signatures are critical in high performance aircraft. With high speed bullets and short time of flight it wouldn't make a lot of difference since all decent bc bullets are aerodynamic. But it could make a few inches when your shooting over 1000 yards. And I bet it is not in the formula. Magnum bullets average +- around 2000 miles per hour and around a second to 1000 yards. So it would not make a lot of difference but it could make some difference. It could be that it is so minimal that it is just not included. Like groper said, I may be just talking out the wrong end but interesting to check anyway.

 

[eddybo]

What exactly is a true BC? Is it the number that works in your ballistics program for all distances, or is it something else. As far as I am concerned whatever goes into exbal is the true BC.

 

[groper]

Not sure where you're talking out of but I used this quick and easy (but accurate) online balistic calc, which I've done before, and it tells me that only two things matter when it comes to Windage, Drop and ToF... and they are velocity and BC. You can change any other parameter, but as long as the BC and velocity remain the same, the Windage, Drop and ToF remain the same.

I know this, this is what the balistics calculators are telling us... but they are only automating the equations... The equations maybe missing some input that causes errors in the windage, maybe they do not... I was simply looking for an explanation as to why LTLR found that when he shot 2 bullets of the same BC, but different calibers and weights, at the same time "side by side" at the range in the same wind, he found very different windage results on the target... i have never done this experiment myself, and have no idea how well he controlled the experiment... i simply said i would not be surprised if what he found was true and offered a theory as to why... i doubt there would be much in it, otherwise the ballistics calculators would make some adjustment based on caliber and weight aswell as BC... it is likely that the effect/difference is so small, that the equations and calculators ignore it just for simplicities sake...

 

[MontanaRifleman]

Montanarifleman, That is my point that if you take the same bc and velocity, etc it will spit out the same results. However I have found discrepancies in that.

LTLR, As we know from Bryan Litz's research, most advertised BC's are inaccurate. so maybe the bullets BC's were actually different. Another question that comes to mind is how steady was the wind you were shooting in? Most windy situations I shoot in, there is a fair amount of gusting and shifting going on as well as up and down drafts. I've seen some strange things shooting in the wind from shot to shot with the same "accurate" load. Not sure what all your variables might have been, but the physics and math don't lie.

 

[WildcatB]

Using JBM's calculators, the velocity method is a lot more error tolerant - especially when dealing with the accuracy tolerances of chronographs. Plugging in the +/-.5% (accuracy tolerance of most chronographs) makes a significant change in the result using the time of flight vs velocity method. The distance measurement is also more crucial than with the velocity method. +/- 1 foot at 300 yards changes the result by 2% using the time method vs .1% (yep that's POINT one percent) using the velocity method.

The same guy also wrote this paper: http://arxiv.org/ftp/physics/papers/0601/0601102.pdf

Once I ran the numbers on the error tolerances… I switched to the velocity method.

I still buy you a beer if you ever come through Elko.

If you want to do a more accurate real world BC calculation, rather than do a drop test, provided you own a laptop computer

try this --> http://arxiv.org/ftp/physics/papers/0601/0601102.pdf

Follow the theory outlined in this paper, we are going to use the same theory for BC calculation rather than velocity calculation. We will shoot thru the chrony for the velocity.

Remember to allow 0.001 seconds for every foot your microphone is from the muzzle AND gong.

Make sure you allow for wind speed in your speed of sound, or simply wait for a no wind day.

Make sure you get the air temperature exact.

Shoot a gong at as much range as possible, the mic of your laptop will pic up the sound of a gong @ 1000yds.

Record each shot velocity and time of flight seperately. Once you have enough data, run several calculations to figure BC, and use the median result for your actual real world BC.

You can thank me with beer

 

[edge]

I'll assume that you have a tapered base, probably at least 20 MOA.

Since I work from drops it does not matter to me, but if I wanted to compute drop from the BC how would I compute the scope height? Is it the height of my reticle above bore? If so then that would change based on a Front or Second focal plane scope!

Just wondering.

edge.

 

[elkaholic]

I'll assume that you have a tapered base, probably at least 20 MOA.

Since I work from drops it does not matter to me, but if I wanted to compute drop from the BC how would I compute the scope height? Is it the height of my reticle above bore? If so then that would change based on a Front or Second focal plane scope!

Just wondering.

edge.

That's the way I do it (centerline above bore) but that does bring up an interesting question about focal planes? Someone smarter than me will have to address that one. I'm guessing it's so slight as to be insignificant, but???...........Rich

 

[Michael Eichele]

First of all, I dont see Bryan as infallible. Sorry Bryan. I have seen several major differences between BC's he has calculated and posted and what I have seen for myself shooting over double chronies and validating them on the range. There are times my results and his have been scary close and others not so close. Remeber, he did publish the 300 grain Hybrid at .891 initially and changed it later to .818 after multiple shooters pointed out the flaw. I dont bring this up to pick on him or beat him up just to illustrate that he is not infallible. I am not saying he is wrong here or right but the reality is probably somewhere in the middle.

I am not syaing he is wrong to do this but he typically publishes an average BC where the BC at 3200 FPS may be .653 but the published number is .624 (just an example). That said, this may be where some of the differences here come into play. Also, how many shooters are using these bullets. If there are 20 or 30, that really isnt enough to form a real good picture of what is going on. If it is 100 or 200, you will get a much better picture.

I am a firm believer in using more than one method for checking or testing BC's and not over one course of fire either. My system is to start out with 2 chronies both lined up front to front to back at 100 yards. Loads are fired over them for comparison purposes. Any discrepencies will be factored into the equasion. Then they are spaced between 15' infront of the muzzle and 300 yards apart. Care is taken so that the bullets are fired over the center of the chronies and at the same height above the chronies. They are also in the same light. One is NOT in the shape while the other is not. Multiple shots are fired over them and recorded. Air density is also recorded. Then it is to the ballistic calculator for computing a BC. Once this is figured out, the rifle is taken to 750 yards. Just before it is fired at 750, a dead on 300 yard zero is verified. Then 750. In every case, when using this method bullets have fallen into the middle of the X ring at 750 yards.

My numbers typically dont match published or Bryan's. Some are close to one or the other and on rare occasions, both. Albiet I realize that Bryan is using an average taken from different muzzle velocities and over a further course of fire so I dont expect them to be exact.

The point is that my methods, Bryans methods, and every body elses methods are going to be different and in different circumstances. There is no way we are all going to match.

I would suggest that Wild Cat bullets at least fire over double chronies and the numbers will speak for themselves.

 

[ZEEK]

Thanks Zeke, I will look at the formulas and see how the wind variable is calculated. My guess is it is straight from the bc because like stated above any bullet with the same bc gets the same results. I know that wind affects different mass and shape objects in different ways. So I will look for the variable for this. In areospace work both the frontal and side signatures are critical in high performance aircraft. With high speed bullets and short time of flight it wouldn't make a lot of difference since all decent bc bullets are aerodynamic. But it could make a few inches when your shooting over 1000 yards. And I bet it is not in the formula. Magnum bullets average +- around 2000 miles per hour and around a second to 1000 yards. So it would not make a lot of difference but it could make some difference. It could be that it is so minimal that it is just not included. Like groper said, I may be just talking out the wrong end but interesting to check anyway.

LTLR,
I found the following statement in an article on exterior ballistics:

Wind has a range of effects, the first being the effect of making the bullet deviate to the side. From a scientific perspective, the "wind pushing on the side of the bullet" is not what causes wind drift. What causes wind drift is drag. Drag makes the bullet turn into the wind, keeping the centre of air pressure on its nose. This causes the nose to be cocked (from your perspective) into the wind, the base is cocked (from your perspective) "downwind." So, (again from your perspective), the drag is pushing the bullet downwind making bullets follow the wind.

If I read this correctly, then what is changed by the wind force, is the yaw angle. To see the variables that the yaw-of-response-vector is a function of see this page:

http://www.nennstiel-ruprecht.de/bullfly/yawrepf.htm

Really interesting physics.

 

[Jon A]

I am not syaing he is wrong to do this but he typically publishes an average BC where the BC at 3200 FPS may be .653 but the published number is .624 (just an example). That said, this may be where some of the differences here come into play.

Of course it is. You are measuring two different things. Why would you expect them to match?

Once this is figured out, the rifle is taken to 750 yards. Just before it is fired at 750, a dead on 300 yard zero is verified. Then 750. In every case, when using this method bullets have fallen into the middle of the X ring at 750 yards.

Drop at 750 from a 300 yds zero is absolutely and completely meaningless as a method of verifying BC's.

The point is that my methods, Bryans methods, and every body elses methods are going to be different and in different circumstances. There is no way we are all going to match.

But they should, if we're all doing it right. Unless there's something wrong with a rifle/bullet combination used by one of the people measuring, if they all do it right they should all get the same answer (within a certain amount of variability).

But like I said, you have to be measuring the same thing. My own measurements have been very close to Bryan's. At first blush they may not be but that's only because mine are relatively incomplete. A G1 BC I've measured over 200 yds certainly won't match his average for its entire flight. But when you look at the BC he measured at the velocity I did, it's usually right in there with what I measured.

With most of the bullets we use, simply using G7's fixes that for the most part as it stays more constant. With some bullets that don't follow that curve or the G1 curve very well, however, you won't get a complete picture unless you measure the bullet over the entire velocity range as he does. That doesn't mean my measurement was wrong, it was just incomplete.

I would suggest that Wild Cat bullets at least fire over double chronies and the numbers will speak for themselves.

He did. And it matched Bryan's numbers almost exactly.