Let's argue about BC's

[eddybo]

Another bone to chew on.... it seems that the longer the barrel the closer to .9 bc g1 is needed to get the drops to match up.

I just emailed a person who shoots them out of a pistol and he needs .75 g1 to get his drops to match.

On JBM's website when you check "Elevation Correction for Zero Range " - the software determines the vertical angle (angle of departure) based on your zero distance. This introduces some error since it doesn't ask for your barrel length. I asked JBM about this. He acknowledged that there was some error here, roughly the barrel lenght multiplied by the vertical angle result. But he thought it was too insignificant to make a difference.

I shot the bullets yesterday at 1000 using .74 with the correct scope height (measured at the chamber) and they still landed 6 to 7 inches high - a .84-.85 put me right on. It's not .9 but not .74 either.

Bryan, you know the equations for this....I couldn't find them anywhere. What exaclty is the error and how much difference in elevation would the corrected departure angle give you at 1000 & 2000 yards using 18, 24, 28, 30, 32 inch barrels?

Just give up not matter what you do they will try and pick apart your methods and claim you must have done something wrong, they do not want to debate the issue by sharing their experiance with any bullet load or chamberiing just act as cheerleaders for the G7 form factor. While stating nothing other than G7 BCs work and since they do the .74 must be correct also. I have never shot any wildcats manufactured since you took the helm, but would not hesitate to do so. I am sure that the quality is still there, and would urge to you to make more 169gr 170 bullets........hint I only have 400 left.

I have measured scope error at long range very similaraly to Bryan's reccomendation to Michael for measuring scope errors. Plus box testing, I have made these adjustments in my ballistics programs seeing errors very in line to the scope errors most see. I find it strange that the same scope that has been accused of dialing inaccurately has shot out to a mile using a G7 BCs accurately. I guess that is some proof that G7s work, but it is of no value to help explain that G1 BCs are very inaccurate, which is my only stance on the subject, and that you gotta put the work in shooting. Although I have seen some very strange stuff with G7s also with same said scope.

I think people would like to think that shooting long range is plug and play. "Oh I do not have to shoot 1500 yards to shoot at an animal at that distance because I use a G7 BC," don't bet on it guys. I do feel that G7 BCs are an improvement, maybe even a big improvement, but they are not a substitute for shooting those ranges. As I stated earlier in this thread, I wonder if those who apparently think that BCs have a great value over a starting point, have shot very much long range. I once felt that same way, until I started finding myself doing a greater percentage of my shooting past 1K.

I have no scientific background, do not care to learn more about the physics behind bullet drop. No offense to the engineers at this site, but something can get over engineered. I cannot give you an accurate BC, I cannot tell you how a ballistics program pumps out its magic info......as it really is magic to me. I can tell you that in the last few years I have burned a lot of powder, wasted a lot of lead time and money to make first round cold bore hits way further than most people even realize is possible, and what I have learbed from those experiances. Nothing can be taken for granted, you have to shoot the distances no matter the BC form factor you use.

Bryan trepidation about BCs is absolutely nothing personal against you. I think you are a great guy who cares a lot about the shooting sports, and based upon my brief dealings with you think you are of the highest moral character, and very dilligent. I just do not have faith in any BC form factor, the more I hear about it the more faith I lose. I want to ask you a very personal question, which you can repond to here or in a PM if you wish to respond at all. Here goes. You have a new rifle custom built for long range hunting. You have taken every conceivable step to prepare yourself for a shot on a big bull elk, including having shot your new stick out to 1000 yards multiple times. Your drop charts are working very well as are your windage reads. You encounter an opportunity for a shot at 1700 yards on a monster bull, what do you do? Getting close is not an option. Pass or take the shot? Do you have the confidence to take the shot at a distance you have never shot with this particular rifle even though you have shot several other rifles at that distance?

BTW your new rifle is shooting a 210gr Berger

 

[BryanLitz]

Eddybo,

I've read all your posts in this thread, and they all say the same thing. To paraphrase:

"There is a lot that I don't know, or care to know about how ballistics programs work and what their sensitivities are to error. I admit to not using the program 'properly'. My primary calculator only works with G1 BC's and I don't really understand or care to understand how G7 BC's are different. By misusing my software, I've noticed that a false BC can be used to match up with things I observed while shooting. Based on that, I conclude that BC's are meaningless, useless, and anyone who expects accurate results with them is naive."

OK, maybe I put it a little more candid than you did, but I believe that's more or less how you feel.

Don't you think it's unfair to discredit parts of how that system works while acknowledging there is a lot going on that you don't understand?

I'll try and make an analogy. The plumbing in my house is a complete mystery to me. I know theres clean water coming in, dirty water going out, and it goes thru pumps, pipes, and filters. Somewhere outside there's a well and a septic tank. So one day one of my pipes starts leaking. I've soldered electronic components before, and I know that copper pipes are soldered together, so I decide to try and solder my pipe to stop the leak. Long story short, after my best efforts, the leak is still there. I conclude: "Even though I'm not an expert at soldering pipes, this should have worked for me. Whoever thinks that pipes can be soldered effectively is naive". Is that a fair conclusion for me to come to? Don't you think a more reasonable conclusion might be: "I wasn't able to effectively solder the pipe with my limited knowledge and skill. However, I acknowledge that someone else, a professional plumber for example, would be able to make that particular technology work because he does it all the time, and has the experience, knowledge and skill to effectively do it right"

Back to ballistics. If you've been able to get your trajectory to line up using dramatically false BC's, then there is some other error in the system that's offsetting that. Some of the things you write seems to show that you know that. So I ask again: how can you say that one component of the system is unreliable when you know that other parts may be in error as well?

If I went out and shot a rifle at long range and observed that my point of impact was really high or low, there's the obvious question: why? One 'quick and easy' thing to do is to fiddle with the BC input for the program until I see what BC is necessary to predict the trajectory that I just observed. If I do that, and conclude that I need a BC that's 20% higher than the BC which is accurate and credible, I look elsewhere for the error. If it only takes a 2% tweak to the BC, ok, that small error could have come from anywhere, but 20%? What planet do you live on to think that a number which is verified accurate within +/-1%, and works for many people is just simply off by 20%, for you?

I totally understand the position of: "I don't care to know why it's off. Using a BC that's 20% different from what it's supposed to be gives me results that I'm happy with". If that's the way you roll, so be it. What gets under my skin is when you get on here and tell people that the entire idea of using BC's to predict trajectories is non-sense when you know you're not doing it right! There are people reading this thread who are here to learn and it would really be a shame if they come away with the idea that ballistics is junk science because eddybo can hit his target with all kinds of non-sense in his exbal.

Having said all that, do I believe that an average shooter can take a new rifle, zero and chrono it at 100 yards, and take it all the way out to the end of it's supersonic range with perfect predictability? Not likely. The reason I don't think it's likely is because I don't have absolute faith in the knowledge, skill, and measurement abilities that the average shooter will apply in the 100 yard shooting. Do I think I, personally, can do it? I think I stand a better chance than most, but I wouldn't bet a million dollars that I'm good enough to center a group at 1000 yards based on 100 yard results.

Here's the key difference between our views: I believe that if I fail at centering the group at long range, that it's because of my imperfect knowledge and/or application of the science. You seem to believe that if you fail to center the group at long range, that it's because the science is actually wrong.

And here's the reason why it's important to understand our different views: My way of looking at it means that if you keep trying to learn and understand ways to reduce the uncertainties, that the underlying science will continue to bring you closer and closer in proportion to how well you've minimized the uncertainties. Your view discourages one to even try to eliminate all the uncertainties.

To answer the question you put to me about the 1700 yard shot; no way in hell I would take the shot. For one thing, the bullet went transonic about 400 yards before the target (3000 fps MV), and I know of at least some cases of the 210 tumbling at transonic speeds (near sea level). Even if it stayed stable, it's going to hit 1700 yards with less than 600 ft-lb of energy, and an impact velocity way below that bullet's ability to produce reliable expansion. Finally, even if you could guarantee 1 MOA accuracy from your rifle (tall order at that range) you're looking at a 17" impact area centered over the vitals. I think our 'monster bull' deserves better than that. So those are my primary reasons for not taking that shot. But I know you trying to see if I have faith that the ballistic solution would be valid. Again my policy is: trust, but verify.

Going out and shooting long range with a fire solution based on 100 yard testing, for me, is an opportunity to learn what mistakes were made in the 100 yard testing. If the POI is 'off' what it should be, something was done wrong.

An exception is if you're shooting into or beyond the transonic range of the bullet. In this case, modeling the drag of a bullet with BC's (G1 or G7) can introduce actual error in the predicted trajectory, and you would need to know the 'custom drag profile' of that particular bullet. However, for the entire supersonic range of the bullet, the G7 BC based trajectory prediction should be 'spot on', as it is in many cases where measurement uncertainty is minimized.

Please don't misinterpret any of my above statements as disrespect to you. Yes, we disagree about how to 'do' ballistics, and honestly, as the ballistician, it's sort of frustrating for me sometimes. However, I have the choice to engage this thread or not. Engaging it in an angry way wouldn't be productive so consider me not-angry

I remember you helping me out when I couldn't get samples of a certain bullet to test. I know you are also passionate about long range shooting and we want the same things. This is just a discussion about our different approaches toward getting there.

-Bryan

BTW, I've never tried to solder copper piping with an electrical soldering iron! That was just a hypothetical analogy.

 

[Buffalobob]

However, for the entire supersonic range of the bullet, the G7 BC based trajectory prediction should be 'spot on', as it is in many cases where measurement uncertainty is minimized.

Let us look at the numbers you put up for the G7 of the Amax in Post #125

What we see is a variation of 10% over the useful velocity of the bullet. Most of that is below the location of the reported average velocity of Wildcat G7. If one takes a near field G7 and compares it to a far field G7 one finds that every one percent change in G7 introduces a one foot miss at 2000 yards. And if one see a 4% change in G7 after the average then one misses ones target by about 4 feet. This is why I use very low BCs for the 200 grain Wildcat. I have very little concern for the first 1200 + yards. My chief concern nowadays is the last 500 -700 yards the bullet will stay stable. The antelope I killed at 1080 the last day before going to Utah was struck very high. If I had been using a higher BC then I would have hit lower at 1080 yds but would have been way low at longer ranges.

 

[eddybo]

I appreciate your reply and understand what you are saying, but please answer this for me. When I use one rifle and two different bullets using g7 bcs how one bc can work very well and another fails pretty badly, using the same equipment and methods.

My only point in this whole thread has been that you have to actually have to shoot the rifle at the ranges you wish to shoot an animal. You did a better job of explaining my point than I have and I thank you for that. Given the level of experience with many on this site I think it is important for shooters to understand that they cannot zero a rifle at 100 yards and be ready to shoot at 1k. Your honest admission that you probably cannot do this and have doubts that the readers here can do it either makes my only point.

Like I said I do not care why something works, I am willing to admit that some are more exacting in certain aspects of their preparation. I also do not have the ego to think that I even have a clue about what goes on when I pull a trigger. What I lack in understanding I try to make up in hard work, by doing lots of shooting at extreme ranges.

Btw I had originally listed the 300gr 338 berger as the bullet you would be using, but thought you might see it as rattling your cage. I should have adjusted the ranges accordingly.

 

[rscott5028]

Just because G1 or G7 BC's aren't perfect doesn't make them useless. In fact, they seem to work pretty well considering how imperfect we recognize them to be and how little the average guy (like me) understands them.

It's also a good idea to have condescending opinions and alternate methods for measuring velocity (chrony vs microphone) and trajectory curves.

As to the latter, Arthur Pejsa wrote an article entotled "Super-Accurate Prediction At Extreme Ranges, Modeling Drag and Calculating Trajectories" that appeared in the July 2010 issue #75 of VARMINT HUNTER magazine.

When numbers don't add up, I always look for a toy or some other method to approach from a different direction in order to corroborate the findings.

Has anyone independently tested Pejsa's theories and how they stack up against traditional BC methods? He may be a genius or a quack for all I know.

thanks,
Richard

 

[BryanLitz]

Bob,

I need to clarify; the BC's listed as a function of velocity that I gave in post #125 are instantaneous BC's that apply for that instantaneous velocity. They are not the cumulative average BC's.

I point this out because it seems like you might be misinterpreting the information.

I'm re-posting the data here, with an additional column. The additional column shows the running, or cumulative averaged BC as the bullet flies downrange.

velocity G7 BC  cumulative average
3300	0.255	
3200	0.254	0.255
3100	0.253	0.254
3000	0.252	0.254
2900	0.250	0.253
2800	0.249	0.252
2700	0.247	0.252
2600	0.245	0.251
2500	0.243	0.250
2400	0.241	0.249
2300	0.239	0.248
2200	0.236	0.247
2100	0.234	0.246
2000	0.232	0.245
1900	0.231	0.244
1800	0.230	0.243
1700	0.230	0.242
1600	0.231	0.242
1500	0.233	0.241
1400	0.243	0.241
1300	0.242	0.241
1200	0.223	0.241

You'll notice that as the bullet traverses instantaneous BC's from .255 at 3300 fps to .223 at 1200 fps (the 10% variation that you pointed out). However, the cumulative average BC, which is what's more important from a trajectory prediction standpoint, the average converges on .241 over long range.

So what error can be expected if you use an average G7 BC of .241 when the actual BC is varying from .255 to .223?

I ran a comparison to demonstrate.

G7 BC average of .241
results =
            0         3250            0      -1.5
          100       3044.1     0.094359         0
          200       2843.7       0.1963   -2.2572
          300       2649.8      0.30558   -8.8203
          400       2463.2      0.42298   -20.343
          500       2284.4      0.54943   -37.605
          600       2113.1      0.68595   -61.542
          700       1949.3      0.83374   -93.280
          800       1791.9      0.99423   -134.19
          900       1640.8       1.1692   -185.96
         1000       1495.5       1.3607   -250.68
Custom drag curve
results =
            0         3250            0      -1.5
          100       3053.7     0.094203         0
          200       2860.9      0.19568   -2.2332
          300       2672.2      0.30415   -8.7202
          400       2487.4      0.42048   -20.084
          500       2307.2      0.54568   -37.078
          600       2131.8      0.68092   -60.617
          700       1961.6      0.82759   -91.824
          800         1797      0.98734   -132.09
          900       1638.6       1.1621   -183.16
         1000       1487.4       1.3543   -247.21

columns are: range, velocity, tof, and drop from a 100 yard zero.

Notice that the error in drop prediction doesn't exceed 1" until after 600 yards, and grows to a maximum of 3.47" at 1000 yards. In order to match this drop at 1000 yards, you would have to use an average G7 BC of .245.

All of this makes your point; except for the magnitude. Going from .241 to .245 is less than 2% difference.

Maybe if the same exercise were repeated with the 7mm 200 grain ULD it would reveal that an even higher average G7 BC is needed to match the 'real' drop at 2000 yards.

I stated earlier that the closer you get to transonic, the worse the match can become to G7 BC's. Maybe that's what's at play. I don't know how fast you guys are shooting these 200 grain bullets, but they've got to be pushing transonic by 2000 yards? At 3000 fps MV, my calculations put it transonic at 1400 to 1600 yards at sea level.

I'm currently working on the second edition of my book. There is a new chapter on Extended Long Range shooting which I define as shooting into/beyond transonic. The chapter addresses some of the unique problems that we're discussing here with drag modeling, as well as stability, challenges and importance of accurate rangefinding, huge scope travel required, sensitivity to consistent MV and bullet-to-bullet BC, etc.

The objective I hope to achieve is to not muddy the water for the 95% of 'normal' long range shooters who work well within the supersonic range of their bullets, and for whom normal averaged G7 BC based predictions work perfectly. For example, it would be a shame for a guy shooting 600 yards to get wrapped around the axle worrying about custom drag curves or coriolis, etc.

eddybo,

Just saw your post. We are on the same page in that we both advocate actually shooting at ranges that you plan to hunt. You have a good point about the experience level of some new LR hunters, and what are in some cases unrealistic expectations taught by TV shows, etc. It takes hard work to achieve consistent, reliable trajectory prediction at long range, but it is possible.

-Bryan

 

[Buffalobob]

I shoot the 200 grain Wildcat at 3350 fps and the bullet is in good enough shape with a clean barrel to get a 0.488 MOA group at 1563 yards for four shots. I can also blow them to dust if I wish by letting the barrel get dirty or hot.

At altitude (5000 feet) I can stay transonic to at least 2200 yards.

 

[Terry Scott]

I would almost always just go with Bryan's data.
I am very intereseted in what gun/cartridge you are using. 190 7mm @3300
YEAH!

 

[BryanLitz]

Holy crap Bob, that's smokin!

I ran a custom drag profile for the 7mm 200 grain ULD, see below:

Vel	G1 BC	G7 BC
4000	0.830	0.349
3900	0.819	0.351
3800	0.809	0.353
3700	0.799	0.355
3600	0.789	0.357
3500	0.779	0.358
3400	0.769	0.360
3300	0.761	0.361
3200	0.754	0.362
3100	0.748	0.362
3000	0.741	0.363
2900	0.735	0.363
2800	0.729	0.364
2700	0.726	0.363
2600	0.723	0.363
2500	0.720	0.362
2400	0.719	0.362
2300	0.718	0.361
2200	0.718	0.361
2100	0.719	0.361
2000	0.719	0.361
1900	0.718	0.363
1800	0.715	0.366
1700	0.710	0.370
1600	0.702	0.375
1500	0.695	0.385
1400	0.686	0.401
1300	0.657	0.415
1200	0.601	0.425
1100	0.641	0.458
1000	0.795	0.343
900	0.631	0.305

Here is the data plotted:

At you're disintegration velocity span, from ~3400 to 1200 fps, the averages are:
G1 = .717
G7 = .371

These are not much different than the average BC's from the 'normal' 3000 to 1500 fps velocity range that I average BC's for (bracketed above).

-Bryan

 

[Buffalobob]

Thanks for the data.

It begins to explain why I can't hit anything with the banded G1 BCs at distances of 1500+ yards. I would never have believed it would decrease so fast and so far from where I know it to be fairly accurate.

We will see how accurate your analysis is in September. Videos will be posted if you are right!!!!

 

[Michael Eichele]

Just because G1 or G7 BC's aren't perfect doesn't make them useless. In fact, they seem to work pretty well considering how imperfect we recognize them to be and how little the average guy (like me) understands them.

It's also a good idea to have condescending opinions and alternate methods for measuring velocity (chrony vs microphone) and trajectory curves.

As to the latter, Arthur Pejsa wrote an article entotled "Super-Accurate Prediction At Extreme Ranges, Modeling Drag and Calculating Trajectories" that appeared in the July 2010 issue #75 of VARMINT HUNTER magazine.

When numbers don't add up, I always look for a toy or some other method to approach from a different direction in order to corroborate the findings.

Has anyone independently tested Pejsa's theories and how they stack up against traditional BC methods? He may be a genius or a quack for all I know.

thanks,
Richard

By tested his theories I assume you mean compare his calculations with a real world shooting scenario?

I am sure to catch flack for this but I will offer it anyways.

I have had limited success using his unmodified formulas despite being close. The principal and idea (IMHO) is genius

All in all, I prefer the calculations based on Pejsa's formulas. I have modified them a bit to my own likeing to better match real world results for my and other shooter's tests and results.

When left alone on the standard default, it will match ANY other widely accepted and produced software using the G1 values. Including, JBM, RSI, Berger, and so on. My personal software uses a BC decay rate value in lieu of a 'drag model' This number can be adjusted up or down to be used in conjunction with a BC. So instead of a G1/.515 BC, it will use a .290/.515 BC. These numbers can be adjusted to match another drag model as well. The big adjustment comes from the BC decay rate where the BC will stay very close to the G1 value. You cant think of it as a G1 curve though because the curve will take on another form. I say G1 because for a raw BC value (without concideration to the drag model) it is the number used for the BC. The trajectory curve will be adjusted with the use of the decay rate. This eliminates the need to input multiple G1 BC's like the Sierra software.

The higher you make the BC decay and BC together, the more you can 'flatten' the mid point trajectory while keeping the long range trajectory the same. The lower each become, the higher the mid point becomes.

See example below:

I hope it makes sense and please keep an open mind.

M

PS, the 1000 yard velocity in JBM (.248G7) and mine using .600/513 are within 14FPS of eachother. In addition, the .248G7 vaue I used came from Bryans postings in this thread. He stated .247 for 2700 and .249 for 2800. Naturally, I used .248 for 2750 for comparison purposes. I am not saying I use these numbers for my trajectory with this load, rather it is to illustrate that it can match with a reasonable degree of accuracy a G7 curve or any other curve for that matter.

Do I think that Pejsa was on to something? He!! yes. Perfect? No. I think the principals behind his method offers the best of both worlds. If a shooters wants a one size fits all G1 or G7 trajectory, he can adjust the BC and decay rate to match. If another wants to use a 'customized' curve, he can do that as well.

 

[rscott5028]

Thanks Michael. Thanks for your post. It'll take me a while to process.

I also see where Pejsa's article was published here as well...
Super-Accurate Prediction At Extreme Ranges Modeling Drag And Calculating Trajectories

The ensuing discussion seems pretty lightweight.

-- richard

 

[Michael Eichele]

Wow. I have never seen that article. It is an excellent read. I may revisit the formulas and see if I can get his raw formulas to match real world data. He cleared a couple of things up for me.

M

 

[Michael Courtney]

First of all, BCs should be measured with either the velocity loss or the time of flight technique. Using bullet drop is prone to many errors because bullet drop is sensitive to many uncertainties and confounding factors that do not exist at all or have much smaller effect on velocity loss and time of flight techniques.

Secondly, anyone who cares about BC accuracy and effects on retained velocity, wind drift, and drop should read Bryan Litz's book.

We've published four papers on BC issues and acoustic measurement techniques:

http://arxiv.org/ftp/physics/papers/0601/0601102.pdf

http://arxiv.org/ftp/arxiv/papers/0812/0812.4752.pdf

http://arxiv.org/ftp/arxiv/papers/0705/0705.0391.pdf

http://arxiv.org/ftp/arxiv/papers/0705/0705.0389.pdf

Even though we've validated an acoustic technique with simultaneous use of near and far chronographs, the two chronograph method is more accurate and highly recommended where possible, because small errors in various measurements (near velocity, distance, temperature, air pressure, etc.) have smaller impact on BC determinations with two chronographs.

In our two chronograph method, we verify that the two chronographs give the expected velocity drop (1-4 fps) when placed a few feet apart before we extend the distance to 300 or 600 feet.

We have seen significant variances in BCs between different rifles, and even within the same rifle before and after polishing the bore. Lots of factors effect bullet drag. In the absence of a BC measurement in a specific rifle, Bryan Litz's numbers are usually the best available estimate, but I strongly prefer to measure the BC in my rifle using two chronographs spaced by 300-600 feet. A single chronograph and our acoustic technique for your specific rifle would also be preferable to just going with any published number. In contrast, you're probably better off going with a published number than with a BC estimate determined from drop.

Michael Courtney