Let's argue about BC's

[LR3]

No dig intended here, but Bryan doesn't use performance to derive BC's??? That's news to me. I think if you look at his methods, he uses "actual" performnce to a much greater degree than anyone else here.

My mistake MR. Should have added bullet impact performance. I m a great fan of Bryan s book and I have studied it very closely. My question was more if there was an alternative to Bryans very excellent method (pg 31 describes the methods and pgs 32 to 35 possible sources of error - muzzle velocity, distance from chronograph to microphones, time of flight to microphones, and air density) since I would not be able to do this for 375s and Michael Courtney answered he was unaware of an alternative program. Clearly his is the best method except perhaps radar and I was pleased to see Bryan come into the conversation given his knowledge. Unless Bryan has published the G7 number one is left somewhat guessing, as in the 375s , and trying to match impact performance to BC. Hopefully he ll have the time to check the new 375s like 414gr GS, and 416 and 458 vld s.

 

[groper]

LR3,
I pulled this from earlier in the thread,... dont forget to read the pdf files mike courtney posted to get the basic theory in your head, then check out my description/modification to the process disclosed below; - its almost as good as bryans method, provided you pay careful attention to the details...

Originally Posted by 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

Thanks for your contribution Michael, ive read your papers and like the ideas presented - however i can see some problems within your methods, and if done differently, would improve the accuracy/resolution of the accoustic method of determining BC - same goes for the 2 chronograph method.

First some of the problems i have with both methods;
1. The testing is not done at far enough distances to provide a good enough AVERAGE of the BC in a long range flight regime - if your only measuring across a distance of 300 or 600 feet, then you can only calculate an average BC in the high velocity part of the flight regime. As we know, the BC decays with decreasing velocity, so we really want to know the average BC over much longer ranges such as 1000yds. So- why not build a steel shield for the second chrony @ 1000yds so we dont smash it, and shoot for averages there in order to get better resolution from much greater velocity decay and better "average BC"? - many dont bother, because its a PITA to setup the second chronograph way out there with sheilds and making sure its actually working before you waste all you ammo etc etc...

Now the same thinking applies to the accoustic method, longer ranges, longer Time of Flight, better resolution, and a more complete flight regime in which to average the BC over. The problem with the method presented in your paper, is with being able to record the sounds accurately - which i guess is why you dont do it at further ranges... So i offer this simple improvement to your method, originally suggested to me by Bryan Litz...

Take a laptop computer and microphone into the pits (where you score and mark the targets protected behind a mound at the dangerous end of the rifle range) of a sanctioned 1000yd rifle range - so you know the distances are accurate. You will also need a 2 way radio at both ends, and have the shooter key the transmitter before he pulls the trigger and shoots thru a chronograph (or 2 chronographs front to back even better) At the recieving end, you get the muzzle blast come thru the 2 way, then some time later the sonic crack of the bullet as it passes overhead. Allowing approx 0.001 sec for each foot the microphone is from the bullet hole in the target, and the transmitter is from the shooters muzzle at the other end, you get a very good Time of Flight measurement over 1000yds -which is usually more than a 1 second ToF depending on the caliber. Reverse calculate the BC using a G7 calculation if your using a VLD type bullet, to match your measured time of flight @ 1000yds and your chronographed velocities. Repeat this 10 times if you wish, in as quick or slow succession as you like, then take a mean or median average, upto you.

Again tho, this is only as good as the operators attention to detail - but it does eliminate much of the errors associated with aiming errors, zero errors, drop errors, scope tracking errors etc - they are completely gone.

The second issue i have is with trusting chronographs. You may have verified the chronies are accurate to each other when placed front to back, but they may be in different light when you shift one out to 300ft - 600ft, and making sure they are perpendicular to the bullet flight etc all contain error... Additionally whilst they may be reading the same, is it an accurate reading or simply that they reading the same inaccurate velocity in those light conditions? Either way, the more distance you have, the better resolution you get and these velocity errors become less influential on the outcome of the calculated average BC...

 

[Michael Courtney]

Magnitudes of BC Variations

I appreciate all the contributions to an interesting discussion. "Significant difference" has two meanings as commonly used. In one sense it means large enough to reduce the probability of success. In this usage, the meaning is subjective to both the expected application and to the values of the shooter. A shooter who cares only about extra drop and not about added wind drift or reduced impact velocity probably cares less than a shooter who would like to be able to make a tough shot on a windier day or values the shorter tracking job that usually results from more reliable expansion due to higher impact velocity.

In its other sense it means "statistically significant difference" which means that the uncertainty in two measured numbers is not larger than the difference between them (the error bars do not overlap). This is the more objective sense in which I most commonly use the word.

How can we better quantify and understand variations in BCs between rifles, between bullets in the same box, and between different lots of bullets? In this post, let's concentrate on variations between bullets in the same box.

A lot of valuable data has been published by Bryan Litz. For example, Bryan's careful measurements have shown that bullets from a box of Berger 155 VLDs with a narrower meplat diameter (0.066") have a BC 2% higher than bullets in the same box with a wider meplat (0.072") and that pointing the tip to 0.053" can increase the BC by about 4% above the nominal meplat.

Even more interesting are the shot-to-shot drag variations in the multitude of graphs in Bryan's book (which everyone with an interest in long range shooting should own and read carefully). Bryan's book has a very informative graph of drag coefficient vs. Mach number for every bullet for which he reports BCs in the book. The ballistic coefficient of any bullet at a given velocity is inversely proportional to drag coefficient, so variations in the measurement of drag coefficient are roughly the same percentage variation in Bryan's measurement of ballistic coefficient. Of course, variations may well result from measurement errors rather than true BC variations, but since Bryan has estimated the accuracy and repeatability of his measurement system at 1%, and since some bullets do show drag coefficients clustered within 1% of each other for nearly the same velocity (Mach number), it is probably safe to suggest that drag coefficient variations more than 1% are most likely due to shot-to-shot variations in the bullets' BC and not to random measurement errors. Therefore, Bryan's graphs of drag coefficient are useful for estimating shot-to-shot variations in BC for bullets from the same box.

One bullet with a lot of shot-to-shot variation is the Nosler .308 caliber 165 grain Partition (p. 488). The graph shows that near Mach 1.5, the drag coefficient varies from roughly 0.41 to 0.49, which is close to 16%. Near Mach 2.1, the variation is from near 0.35 to 0.42, also close to 16%. Of course, the standard deviation is smaller than the extreme spread, but harder to estimate from a graph. Match bullets tend to show smaller BC variations than lead tipped hunting bullets. For example, near Mach 1.75, the drag coefficient of the Berger .264 caliber 140 grain VLD (p. 402) varies from roughly 0.27 to 0.31 which is about 13%. (Of course, a better way to analyze these deviations is to compute the standard error from the best-fit drag model, because this incorporates the deviation from the best-fit drag model of every point in the data set. Another good approach would be to compute the standard deviation of BCs determined for each shot. However, these methods require access to the raw data in tabular rather than graphical form.) Of course, a lot of bullets show smaller BC variations than these, and I have not done sufficient analysis to determine whether these are typical shot-to-shot variations for most different bullets or whether these are two boxes of bullets demonstrating particularly large shot-to-shot variations. I have shown that some boxes of bullets show significant shot-to-shot variations in BC.

Michael

 

[rscott5028]

Michael,

"Significant difference" has two meanings as commonly used.

Great points and explanations.

How can we better quantify and understand variations in BCs between rifles, between bullets in the same box, and between different lots of bullets? In this post, let's concentrate on variations between bullets in the same box.

Can't wait for the follow up post! ...a high quality statistical model that isolates the amount of variation in BC attributable to the rifle... Can you do that for the top 3 or 5 factory rifles so that I know which one to buy?

My point in mentioning statistics was simply that I see claims made in these forums all the time that are based on small samples. That's often great information for identifying a trend or developing a hypothesis. But, you hit the nail on the head when you stated that "extraordinary claims require extraordinary evidence."

Thanks!
Richard

 

[LR3]

Thanks Grouper for your post. The articles are very interesting and since I have access to a 1000yd I d like to try your method sometime. Will be interesting to look at the two 375rifles with same 350smks. May even setup a second chronograph at target to cross check.

 

[Michael Courtney]

Magnitudes of BC Variations with Different Rifles and Lot Numbers

In my last post, I discussed published data (Litz) showing that a single box of bullets can exhibit shot-to-shot variations in BC above 10%. We've seen the same thing in both our published and unpublished data. Sometimes a single box of bullets exhibits small (1-2%) shot-to-shot variations in BC; sometimes the shot-to-shot variations are much larger (10-16%).

Published data where BC is presented for several different rifles and the same lot of bullets is harder to come by. Likewise, I don't know of much published data for the same rifle measuring the BC for several different lots of bullets manufactured years apart to determine with confidence how much the BC of a given model of bullet might be changing over time. The best we can infer from published data includes effects of both different rifles and different lot numbers.

Before comparing data from different sources to speak to this question, care is needed to ensure one has a high level of confidence in the accuracy of the published numbers from the sources, as well as to ensure that both sources are stating their BC for the same velocity, since we know that BC can exhibit significant changes with velocity. Here, we'll discuss variations between Bryan Litz's and Sierra's published numbers for their .224 and .308 caliber bullets where both have published BCs at 3000 fps. Since in most cases, Sierra published their numbers many years before Bryan, I think it is safe to say that in most (if not all) cases, Sierra's numbers result from testing different lot numbers in different rifle barrels from Bryan's tests.

In the .224 caliber bullets for which they both report G1 BC measurements, Bryan reports a BC measurement 14% higher (0.350) than Sierra (0.301) for the 69 grain SMK. In contrast, Bryan reports a BC measurement 3% lower (0.218) than Sierra (0.225) for the 52 grain SMK. Measurements for the other .224 caliber bullets are closer.

Similar variations in reported BCs exist for the .308 caliber bullets for which they both report G1 BC measurements good at 3000 fps. Bryan Litz reports a BC measurement 10% higher (0.448) than Sierra (0.404) for the 165 grain GameKing at 3000 fps. In contrast, Bryan reports a BC measurement 11% lower (0.639) than Sierra (0.711) for the 240 SMK.

We have not measured as wide a variety of bullets as Bryan Litz or Sierra bullets, but our (published and unpublished) BCs suggest that variations between different rifles (same lot of bullets) and variations between different lots (same rifle) can both exceed 10%.

Michael

 

[rscott5028]

Michael,

Thanks for another post loaded with quality information.

I particularly appreciate that you don't immediately assume that either Bryan nor Sierra was wrong in determining different BCs for the same bullet make/model. Or, that they were necessarily flawed in their testing procedures.

Yet, you point out likely reasons for the variations while not asserting those possibilities as facts when statistical data is lacking.

It's nice to know that Wildcat, Berger, and others are aware that there's interest from the growing LRH crowd in continued product development with ever increasing reach and that a published BC won't go uncontested.

Thanks,
Richard

 

[BryanLitz]

Even more interesting are the shot-to-shot drag variations in the multitude of graphs in Bryan's book

In my last post, I discussed published data (Litz) showing that a single box of bullets can exhibit shot-to-shot variations in BC above 10%.

There are several possible causes for the data scatter shown on my plots which don't necessarily mean actual differences in shot-to-shot BC. Refer to pages 288 & 289 of my book for a detailed discussion of this. Below is a summary.

One possible reason is 'messy' test data, usually from static in the sound files that makes it hard to identify precisely when a sound event begins. This is a way for the data to be scattered, even by up to +/-5% or more on the plot, but not actually indicate a difference in bullet-to-bullet BC that large. In other words, in rare cases the measurement resolution is too poor for individual shots to be measured precisely, and the average of the measurements for all the shots is the only valuable result of the test. In other words, each measurement might have up to +/-5% error, but the average is likely to have less error than each data point.

Another likely cause of 'messy' test data is chronograph error, as in the case noted at the bottom of page 338. Small caliber, long pointy bullets like the .224 caliber 90 grain VLD usually have more measured shot to shot variation in MV because the chrono screens work from shadows. A narrow bullet flying over the screens will trigger the light screens less consistently than wider, blunter bullets. Almost all MV testing that I've done with .224 cal, especially the long nosed bullets, indicate higher variation in MV compared to larger caliber bullets. My belief is that a great deal of the perceived variation is actually chrono error.

So the above two mechanisms are both ways that the data scatter in my plots can make it look like there are large shot-to-shot variations in BC from the same lot and rifle, when really it's poor measurement resolution.

The only case I believe there can actually be large shot-to-shot variation in the BC of bullets within a box (more than 3%) is for lead tip bullets. I'm not 100% sure why, but my working theory is that the exposed lead gets deformed (more or less) as the bullet is being fired, and that causes an actual significant difference in the drag the bullet flies with.

I don't believe that typical open tip or plastic tipped bullets will have variations of more than 1% or 2% in BC within a box, and usually not more than 5% between lots. My testing of bullets from different lots has been minimal, but there is one documented case (pg 381) where two lots of Hornady's 6mm 87 grain BTHP from 11 years apart were tested and found to have BC's within 1% of each other! Granted that's anecdotal, and probably not all lots of all bullets from all manufacturers are that repeatable.

Sometimes a single box of bullets exhibits small (1-2%) shot-to-shot variations in BC; sometimes the shot-to-shot variations are much larger (10-16%).

I would be interested in what tests/bullets produced variations on the order of 10-16%. My guess would be there were either small caliber, and/or sunny conditions (known to cause chrono inconsistencies), and/or lead tipped bullets.

MC pointed out above some of the worst case disagreements between my BC measurements and Sierras. It should be noted that on average, the errors are usually much smaller. I think that for all brands tested, Sierra had the least error in their BC's compared to my measurements (not counting 7mm which seems to have a systematic error). The unfortunate thing about Sierra's banded BC approach is that many shooters think that the only BC that applies is the one closes to their MV, so they use the highest BC for their whole trajectory and incur way more error than they would if they understood to use the average. Or maybe shooters just like to be optimistic. I've seen many many times where Berger's BC's, which are clearly stated as averages from 3000 to 1500 fps, are compared to Sierras highest velocity BC by someone who doesn't understand the velocity dependence.

Back to the point...

We have not measured as wide a variety of bullets as Bryan Litz or Sierra bullets, but our (published and unpublished) BCs suggest that variations between different rifles (same lot of bullets) and variations between different lots (same rifle) can both exceed 10%.

I believe that it is possible for a rifle to fire a bullet with a BC that's less than it's potential max value by up to 10% or even more if there's a problem of some kind with the rifle.

I don't believe it's possible for a rifle to fire a bullet with a BC that's 10% higher than it's potential max value from other rifles.

With the exception of lead tipped bullets, or unusually poor quality bullets, I don't believe bullets within a given lot/box will have actual differences in BC's of more than a couple %. Differences between lots might get up to 5% (+/- 2.5%).

-Bryan

 

[MontanaRifleman]

A lot of interesting stuff here Michael.

Magnitudes of BC Variations

I appreciate all the contributions to an interesting discussion. "Significant difference" has two meanings as commonly used. In one sense it means large enough to reduce the probability of success. In this usage, the meaning is subjective to both the expected application and to the values of the shooter. A shooter who cares only about extra drop and not about added wind drift or reduced impact velocity probably cares less than a shooter who would like to be able to make a tough shot on a windier day or values the shorter tracking job that usually results from more reliable expansion due to higher impact velocity.

In its other sense it means "statistically significant difference" which means that the uncertainty in two measured numbers is not larger than the difference between them (the error bars do not overlap). This is the more objective sense in which I most commonly use the word.

Good definitions and clarification. I think for the purpose of the LR hunter, the level of significance will be defined by by how much deviation it takes to put his bullet outside the kill zone and that will change from situation to situation. Not debating, just saying...

How can we better quantify and understand variations in BCs between rifles, between bullets in the same box, and between different lots of bullets? In this post, let's concentrate on variations between bullets in the same box.

A lot of valuable data has been published by Bryan Litz. For example, Bryan's careful measurements have shown that bullets from a box of Berger 155 VLDs with a narrower meplat diameter (0.066") have a BC 2% higher than bullets in the same box with a wider meplat (0.072") and that pointing the tip to 0.053" can increase the BC by about 4% above the nominal meplat.

I find this very interesting. I opened my calipers up to .019" and that's not a big gap. Sounds pretty incredible that it would affect BC by 4%. That would be the difference between a BC of .6 and .576, which would amount to about 5" of drop over 1000 yds with a 3000 fps bullet. That's a lot, and that suggests that I better measure each bullets meplat and for that matter ogive, bearing surface, length, etc., because it would be hard to pick that up by visual inspection. And then when you start to factor other things like load variance, it's a wonder we can hit a 2' disk @ 1K. How did you arrive @ 4% difference... your calcs... Bryan's calcs.... observed result from shooting? I'm going to remain skeptical, because actual results don't "seem" to reflect that.

Even more interesting are the shot-to-shot drag variations in the multitude of graphs in Bryan's book (which everyone with an interest in long range shooting should own and read carefully). Bryan's book has a very informative graph of drag coefficient vs. Mach number for every bullet for which he reports BCs in the book. The ballistic coefficient of any bullet at a given velocity is inversely proportional to drag coefficient, so variations in the measurement of drag coefficient are roughly the same percentage variation in Bryan's measurement of ballistic coefficient. Of course, variations may well result from measurement errors rather than true BC variations, but since Bryan has estimated the accuracy and repeatability of his measurement system at 1%, and since some bullets do show drag coefficients clustered within 1% of each other for nearly the same velocity (Mach number), it is probably safe to suggest that drag coefficient variations more than 1% are most likely due to shot-to-shot variations in the bullets' BC and not to random measurement errors. Therefore, Bryan's graphs of drag coefficient are useful for estimating shot-to-shot variations in BC for bullets from the same box.

I think it would be very hard to assign the cause of 1% of variation. It could be one or a combination of a lot of things including equipment and conditions. never-the-less, I won't say that there might be 1-2% BC variation from bullet to bullet with carefully manufactured match grade bullets.

One bullet with a lot of shot-to-shot variation is the Nosler .308 caliber 165 grain Partition (p. 488). The graph shows that near Mach 1.5, the drag coefficient varies from roughly 0.41 to 0.49, which is close to 16%. Near Mach 2.1, the variation is from near 0.35 to 0.42, also close to 16%. Of course, the standard deviation is smaller than the extreme spread, but harder to estimate from a graph.

I don't think the Nosler Partition really belongs in this discussion. It's not the low drag type of bullet most LR shooters use and it has an easily deformed lead nose. I've read some speculation that the nose might actually melt a little inflight due to heat friction.

Match bullets tend to show smaller BC variations than lead tipped hunting bullets. For example, near Mach 1.75, the drag coefficient of the Berger .264 caliber 140 grain VLD (p. 402) varies from roughly 0.27 to 0.31 which is about 13%. (Of course, a better way to analyze these deviations is to compute the standard error from the best-fit drag model, because this incorporates the deviation from the best-fit drag model of every point in the data set. Another good approach would be to compute the standard deviation of BCs determined for each shot. However, these methods require access to the raw data in tabular rather than graphical form.) Of course, a lot of bullets show smaller BC variations than these, and I have not done sufficient analysis to determine whether these are typical shot-to-shot variations for most different bullets or whether these are two boxes of bullets demonstrating particularly large shot-to-shot variations. I have shown that some boxes of bullets show significant shot-to-shot variations in BC.

Michael

If bullets vary 13%, then we have no business shooting then at animals @ 1K and farther. The 13% alone is good enough for a bad hit or miss let alone all the other factors.

 

[MontanaRifleman]

Magnitudes of BC Variations with Different Rifles and Lot Numbers

In my last post, I discussed published data (Litz) showing that a single box of bullets can exhibit shot-to-shot variations in BC above 10%. We've seen the same thing in both our published and unpublished data. Sometimes a single box of bullets exhibits small (1-2%) shot-to-shot variations in BC; sometimes the shot-to-shot variations are much larger (10-16%).

Published data where BC is presented for several different rifles and the same lot of bullets is harder to come by. Likewise, I don't know of much published data for the same rifle measuring the BC for several different lots of bullets manufactured years apart to determine with confidence how much the BC of a given model of bullet might be changing over time. The best we can infer from published data includes effects of both different rifles and different lot numbers.

Before comparing data from different sources to speak to this question, care is needed to ensure one has a high level of confidence in the accuracy of the published numbers from the sources, as well as to ensure that both sources are stating their BC for the same velocity, since we know that BC can exhibit significant changes with velocity. Here, we'll discuss variations between Bryan Litz's and Sierra's published numbers for their .224 and .308 caliber bullets where both have published BCs at 3000 fps. Since in most cases, Sierra published their numbers many years before Bryan, I think it is safe to say that in most (if not all) cases, Sierra's numbers result from testing different lot numbers in different rifle barrels from Bryan's tests.

In the .224 caliber bullets for which they both report G1 BC measurements, Bryan reports a BC measurement 14% higher (0.350) than Sierra (0.301) for the 69 grain SMK. In contrast, Bryan reports a BC measurement 3% lower (0.218) than Sierra (0.225) for the 52 grain SMK. Measurements for the other .224 caliber bullets are closer.

Similar variations in reported BCs exist for the .308 caliber bullets for which they both report G1 BC measurements good at 3000 fps. Bryan Litz reports a BC measurement 10% higher (0.448) than Sierra (0.404) for the 165 grain GameKing at 3000 fps. In contrast, Bryan reports a BC measurement 11% lower (0.639) than Sierra (0.711) for the 240 SMK.

We have not measured as wide a variety of bullets as Bryan Litz or Sierra bullets, but our (published and unpublished) BCs suggest that variations between different rifles (same lot of bullets) and variations between different lots (same rifle) can both exceed 10%.

Michael

If this is true, it means we are slinging hail Mary's at critter @ 1K and beyond.

 

[rscott5028]

Here I go beating the dead horse for the last time and then, I'm bowing out... (again)

I agree with MontanaRifleman that the "level of significance" in terms of hitting the kill zone is what LR Hunter's are concerned with.

But, I'm just saying that if one's data isn't "statistically significant," then the "level of significance" is suspect and one should gather more data before drawing conclusions and making too many adjustments.

A single two shot 1/4 MOA group doesn't constitute a 1/4 MOA rifle. Or, does it? There sure are a lot of 1/4 MOA fatory built rifle claims floating around in internet forums.

As MR and Bryan pointed out, Michael chose some perhaps extreme cases to illustrate a point or two. But, he was careful not to say that they were "statistically significant".

Conversely, because they may not be "statistically significant" does not render the information useless. Rather, it's an indication that something is possible and may point to a trend that requires further study.

If there was a 4% difference or even a 10 or 16% difference within a given data set, then that would be a fact that can be derived from the raw data using simple math. The cause of which may be attributable to the meplat diameter, shape, and/or other factors including multiple factors stacking up.

The fact that meplat trimmers are in use by some indicates that some marksmen beleive it's worthwhile. Or worst case, that it's of no negative consequence in the quest for uniformity/consistency. If it yeilds a BC improvement of X% and reduces the standard deviation of the BC by Y, then one question becomes a matter of what would it cost the maker to uniform meplats vs what would the customer be willing to pay?

I think Bryan addressed a number of important factors for variability that Michael observed in each other's data citing the reality of 'messy' test data and reasons for that to occur. ...which is precisely why we need the data to be "statistically significant" before drawing conclusions with some level of confidence.

--richard

 

[groper]

The "significance" is important because it defines how much error we can expect in a single long range cold bore shot placement - whats important to us.

If one was to put a 1/2moa aiming error, then misread the wind by another 1/2moa, then shoot a bullet with a 10% BC loss- or a 10% BC estimation error in trajectory - you can see where this is going cant you? This is another reason, the BC should be as close to a "best cumulative average" figure you can get. - to maximize your chances and minimize your errors.

Slight Trimming of meplats uniforms your bullets and therefore uniforms BC - its worth doing IF your bullets have inconsistent meplat diameters... it has a more noticeable effect in the small calibers where the meplat dia is a larger % of the caliber diameter.

 

[Michael Eichele]

A
I find this very interesting. I opened my calipers up to .019" and that's not a big gap. Sounds pretty incredible that it would affect BC by 4%. That would be the difference between a BC of .6 and .576, which would amount to about 5" of drop over 1000 yds with a 3000 fps bullet. That's a lot, and that suggests that I better measure each bullets meplat and for that matter ogive, bearing surface, length, etc., because it would be hard to pick that up by visual inspection. And then when you start to factor other things like load variance, it's a wonder we can hit a 2' disk @ 1K. How did you arrive @ 4% difference... your calcs... Bryan's calcs.... observed result from shooting? I'm going to remain skeptical, because actual results don't "seem" to reflect that.

On that subject, I had opend up some meplats (+.010") on the 190 SMK's a few years ago in an attempt to help a very poor expanding bullet (at least at 308 velocities) expand more reliably. After loading them up to see if the accuracy would deminish or get better, I fired them at 650 yards and found the group center was 15" below the center of the target. I was blown away. I did check my 300 yard zero and made a slight adjustment and went back to 650 yards. They were 11-12" low at that point. I did not have my chrony so I went home to get it and load up some that were not trimmed. I fired both sets over the chrony and found them to all be within normal velocities for that load. I fired the untrimmed loads at 300 yards, rezeroed (1 click), fired them at 650 right into the center of the target. Next I fired the trimmed versions and they were about 13ish" low. Re-zeroed at 300 and again, 11-12" low.

Now I am not going to throw around BC numbers etc.....I am not going to say my BC changed 'X' amount. You guys seem to have a handle on what makes a BC a BC and an observation an observation so I wont bore you with what I think. You can crunch the numbers and theorize for yourselves.

All I can say is that 300 yard zero for zero, the trimmed version was 11-12" below the non-trimmed version all else being equal. Muzzle velocity average was 2660 which was consistent with other velocity tests prior. Accuracy was the same 1/2 MOA for each load. The facts are that the 'observation' was 11-12" of difference between trimmed meplats and factory. Same lot to boot.

M

 

[Topshot]

I had an interesting play with modifying meplat size a while back.
With the new jacket Hornady BTHP match bullets in 250gn .338 caliber.

They made changes to the jacket that made the bullet work better on targets but a poor performer on game. So wanting to keep the excellent ballistic performance of this bullet and improve its game getting performance. I had a go a modifying the meplat.

The idea was to hone out the inside of the meplat with a sharp pointed diamond hone to a depth of 8mm. I found that at this depth the inside hole of the hollow point was about 0.020" wider but the outside diameter was untouched.

So with my way of thinking I would end up with a larger hole and thinner jacket at the pointy end. All good things to promote expansion on Game. I also figured that as the outside of the meplat was unchanged, the modification would have little effect on the B.C. of the bullet.

Man,I was onto something I thought! Should make the ballistic hall of fame with this bit of genius thinking.

Problem was when shot side by side against unmodified bullets out of the same rifle. They shot about 14" low at 1000 yards. So much for making it into the hall of fame. Oh well back to the drawing board

So obviously the inside diameter of the meplat has an effect on B.C. as well as the outside diameter of the meplat. Funny thing that only the outside diameter is ever mentioned when people talk about such things.