Verifying BC

[Toby Bradshaw]

Since it seems that bullet manufacturers routinely provide an "optimistic" BC value for their bullets (see attached pdf), who among us verifies BC by actual testing? For instance, in the attached paper 25 of 27 bullets tested had measured BCs lower that the BC given by the manufacturer -- sometimes shockingly lower. What have you found?

Further, has anyone tested the same lot of bullets in different rifles to see if the rifle itself affects measured BC, as suggested by the authors of the attached paper in the last paragraph of the Discussion?

 

[CharlieNC]

When available I use the Litz published values, which work well once I have sufficient data from various ambient conditions to tweak the velocity. For the newer rdf and eld bullets the producer supplied bc values have proven accurate for me; reports suggest they have become more diligent in this effort, realizing consequences of previous errors. A few months ago I read a Hornady publication on snipers hide showing the impact of twist, barrel length, etc on bc for one of their new eld bullets; it was eye opening.

 

[Brians356]

If different rifles / barrels can account for up to 20% (as suggested), then it largely lets the manufactures off the hook. Only 5 of the 27 bullets tested differed more than 20% from the published BC.

PS

Intuitively, it seems likely that individual barrels would affect a particular bullet's BC significantly. I suppose 20% as suggested by the authors is not [corrected] unreasonable.
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[Toby Bradshaw]

When available I use the Litz published values, which work well once I have sufficient data from various ambient conditions to tweak the velocity. For the newer rdf and eld bullets the producer supplied bc values have proven accurate for me; reports suggest they have become more diligent in this effort, realizing consequences of previous errors. A few months ago I read a Hornady publication on snipers hide showing the impact of twist, barrel length, etc on bc for one of their new eld bullets; it was eye opening.

Hi Charlie. Can you post a link? I Googled around but couldn't find it.

 

[memilanuk]

Hi Charlie. Can you post a link? I Googled around but couldn't find it.

Not that particular document, but I believe chapter 2 of 'Modern Advancements in Long Range Shooting Vol. II discusses the effect of twist rate on BC.

Also shown graphically in this post...

 

[Ned Ludd]

Toby, the paper you referenced in your OP is from a guy named Michael Courtney, who tried his very best to publicly denigrate Bryan Litz and Berger Bullets in regard to published BC values here, and on a number of other internet shooting forums. Believe whatever values you wish, but I would strongly suggest taking his motive into account when you read things like this.

The bottom line is that BCs can change from Lot to Lot of bullets, and can even differ within a single box, due to length variance. The real question is, how fine an increment can you shoot? If you can't shoot the difference, it might as well not be there. Most shooters don't have the proper setup to precisely measure BCs, and will commonly use drop at a known distance to make a crude estimate. I do this routinely and if the estimate is within the limitation of precision of the particular setup I'm testing at a given distance, that's all I need to know. If not, I'll adjust my "working" BC value accordingly. Too many other variables including ambient temperature and velocity can change during the course of even a few hours to even render a standardized BC value the "limiting source of error". For that reason, no BC number printed on a box of bullets or listed on an internet website will ever be better than actual drop data for a given setup. If you're doing a type of shooting that requires long range (cold bore) shots at unknown distances where you don't have solid drop data, I'd agree that the better the BC value you have, the better your chance of making first round hits. For most, BCs published by the manufacturer and tested/validated occasionally by the shooter will be more than sufficient.

 

[284winner]

Toby, the paper you referenced in your OP is from a guy named Michael Courtney, who tried his very best to publicly denigrate Bryan Litz and Berger Bullets in regard to published BC values here, and on a number of other internet shooting forums. Believe whatever values you wish, but I would strongly suggest taking his motive into account when you read things like this.

The bottom line is that BCs can change from Lot to Lot of bullets, and can even differ within a single box, due to length variance. The real question is, how fine an increment can you shoot? If you can't shoot the difference, it might as well not be there. Most shooters don't have the proper setup to precisely measure BCs, and will commonly use drop at a known distance to make a crude estimate. I do this routinely and if the estimate is within the limitation of precision of the particular setup I'm testing at a given distance, that's all I need to know. If not, I'll adjust my "working" BC value accordingly. Too many other variables including ambient temperature and velocity can change during the course of even a few hours to even render a standardized BC value the "limiting source of error". For that reason, no BC number printed on a box of bullets or listed on an internet website will ever be better than actual drop data for a given setup. If you're doing a type of shooting that requires long range (cold bore) shots at unknown distances where you don't have solid drop data, I'd agree that the better the BC value you have, the better your chance of making first round hits. For most, BCs published by the manufacturer and tested/validated occasionally by the shooter will be more than sufficient.

If all the Ballistic requirements are known on a Ballistic program except the "exact" BC, I think that number can be manually adjusted based on impact exactly what your drop is with that bullet and therefore identifying the BC fairly well. Most times better than the manufacturer. Well put by the way as to the explanation of this very controversial topic.

 

[Toby Bradshaw]

Most shooters don't have the proper setup to precisely measure BCs, and will commonly use drop at a known distance to make a crude estimate. I do this routinely and if the estimate is within the limitation of precision of the particular setup I'm testing at a given distance, that's all I need to know. If not, I'll adjust my "working" BC value accordingly.

So what is a typical range of "adjustment" in your experience? Is the typical "adjustment" up or down? And are you correcting for air density when making adjustments to the working BC?

I read the back-and-forth in the Courtney-Litz feud awhile back. It looks like they settled their differences to the benefit of all (unless someone is looking for a simple solution to a complex problem).

I think it's significant that when BCs are revised by manufacturers, they are almost always revised downward. No doubt some of this enhanced honesty is the result of shooters being able to estimate BCs (or at least drops) for themselves, and not just take the number on the box at face value.

Anyhow, one of the Litz responses attached for perspective.

 

[Ned Ludd]

Part of that depends on the distance. In my hands using JBM Ballistics, I sometimes cannot make the trajectory (drop) match exactly for around 600 yd when it is spot on at 100-400 yd and out at 1000 yd. In that event, I usually go with the box BC and leave well enough alone, although I do record the difference. I have used the box BC values for matches at 1000 yd and been very, very close...any difference likely attributed to atmospherics or me. On occasion, I have found that the box value was slightly overestimated for a particular Lot of bullets. However, we're talking pretty small differences, perhaps half to 3/4 MOA low at 1000 yd. That translates to somewhere between a 5-10% difference. Honestly, if the difference was anything less than that and I hadn't shot at those specific ranges so many times under various atmospheric conditions, I'm not sure I'd even be willing to say there was a discernible difference.

I do the same type of analysis/record-keeping with pointed/unpointed bullets, and typically find a 3-5% increase in BC after pointing. That is much easier to measure/compare as with otherwise identical rounds, the ones with pointed bullets always require less elevation to hold target center, so simple subtraction. I shoot F-TR and for the most part, I really need only hit the aiming black on the first sighter. Truth be told, it's always much better if you're inside the 9-ring to start out, as the adjustment increment will be much smaller. However, anywhere in the black and it's not too tough to dial in to center on the 2nd sighter (or 3rd, 4th, etc., depending). So my requirements for BC "accuracy" may not be nearly as strict as others. Nonetheless, I like to keep as accurate records as possible because you never know when it will make a difference, even in the type shooting I do. For example, when shooting matches at Raton, NM (at 6600 ft elevation) coming from San Diego, CA (at about 400 ft elevation), the difference in elevation required at 1000 yd was huge.

In general, the better your numbers are to start with, the better your odds of not missing the target completely. So I do try to keep/use the best numbers possible. But there is a realistic limit as to how much variance in BC would actually have to be present before I could say with any confidence that it was actually different based on shooting results. I have switched bullet types in the past where the BC increase of the new bullet was only about 5% or so. I don't place a whole confidence in using drop data to corroborate that difference. It's there, and you can usually detect it, but it's right at the limit IMO. The primary way I can perceive the higher BC bullet really does have a higher BC is checking my match scores over some time period. You can usually get a feel for the better bullet over time in terms of better scores, i.e. fewer points lost to wind under comparable conditions. However, that is also only anecdotal.

 

[Berger.Fan222]

The LabRadar reports velocity readings out to 100 yards, so if one has accurate atmospheric readings (as from a Kestrel), it's pretty easy to figure out the BC of a given bullet in your actual rifle. My buddies and I have seen some BCs higher than advertised and some lower. Sometimes there is a clear relationship with twist (barely stable loses a lot of BC). Other times there is no clear reason. Some bullets just have higher BCs in some barrels.

Attempting to verify BCs with bullet drop is problematic for several reasons. One can never eliminate the possibilities of vertical air currents, lift, and other confounding factors over 600-1000 yards. Unless one measures the muzzle velocity on every shot, one is mixing in variations in muzzle velocity with both the average BC and the shot to shot variations in BC.

But for me, even more important than a high BC is a consistent BC. Bullets with 5-10% shot-to-shot variations in BC are going to give more vertical spread than bullets with 1% shot-to-shot variations in BC.

I won my first long range precision rifle match on a very windy day with a bullet with a BC of about 0.3. My score sucked, but every one else's score sucked worse. It rained, it snowed, it sleeted, there was a heavy fog and we were shooting into a rising sun. I won a more recent F-Class match on a very calm day with a bullet with a BC under 0.5 or so. Most of the shooters I beat were shooting heavier, higher BC bullets. Sound fundamentals, shooting well in sweltering heat, and a bit of luck were key that day. Good thing Brian Bowling was somewhere else.

 

[CharlieNC]

Hi Charlie. Can you post a link? I Googled around but couldn't find it.

I think it was on the previous hosting site, and the current one does not access those older posts. Maybe on Hornady site, I'll check?

 

[SheepDog]

For me, there is no reason to verify the BC of a bullet. Once I sight a round in I just shoot it at different ranges to build dope for that round in my gun. The actual BC will change with conditions so you would have to "approximate" the BC under the condition at the time of shooting. It takes less time to just shoot and record the dope. Sure, I use ballistic software (I have 8 different ones to choose from) but none of them will give me as precise information as I can get from just shooting a bullet. It is better to keep a dope sheet than to have a calculator. I use them to figure out a point blank range within a 2" target zone then I test it and record my findings.

 

[Berger.Fan222]

For me, there is no reason to verify the BC of a bullet. Once I sight a round in I just shoot it at different ranges to build dope for that round in my gun. The actual BC will change with conditions so you would have to "approximate" the BC under the condition at the time of shooting. It takes less time to just shoot and record the dope. Sure, I use ballistic software (I have 8 different ones to choose from) but none of them will give me as precise information as I can get from just shooting a bullet. It is better to keep a dope sheet than to have a calculator. I use them to figure out a point blank range within a 2" target zone then I test it and record my findings.

Since I introduces lots of new shooters to the sport, they often want to shoot their favorite loads that may be at risk of being subsonic at the target. Consider the new AR Tactical mid-range NRA High Power discipline. The intent is for new shooters to shoot short barreled ARs at 600 yards. Lots of .223 loads (especially light bullets) are in danger of going subsonic on the way to 600 yards when fired from 16-20" barrels. Checking the muzzle velocity and BC with the LabRadar will allow me to tell them whether their favored load will reach the target comfortably supersonic, is marginal, or will go subsonic well before the target.

The same is true of lots of loads new shooters may want to shoot at 1000 yards. Combined with differences from expectations in muzzle velocity, differences in BC from what is written on the box can be the difference between going through the sonic transition before reaching 1000 yards. Verifying the muzzle velocity and BC can save some frustration on match day, especially if one has to travel some distance to get to that 1000 yard event.

Hunters are also concerned with terminal performance (bullet expansion and impact energy) rather than making holes in paper. There is no substitute for measuring muzzle velocity and BC to ensure the bullet remains above its expansion threshold out the the intended ranges.

 

[Brians356]

... enhanced honesty ...

Noice! There's a useful term for a scarce commodity.
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[Ballisticboy]

Over the last ten years I have tracked over 50 combinations of guns and bullets using doppler tracking radar as well as multiple muzzle velocity radars. Using fixed stands, each gun bullet combination is fired on multiple different occasions with 10 rounds in each serial. The met is measured carefully and monitored with strict maximum values for wind speeds. The bullet is tracked for as long as possible, but for at least 10 seconds. I then have to analyse all the data mainly for drag coefficients as we do not use BCs. I cannot report on individual results but I have seen drag coefficient values vary by as much as +/- 16% in some combinations which would of course give differences of 32% between the highest and lowest BC values. Remember this is from one gun firing ammunition from one batch. Some combinations gave +/- 8% while the best were down to only 1-2% variations.
Now if you take the worst of the combinations with values which can vary by over 20% then it is easy to see how individuals firing only 4 or 5 rounds could get large differences between their data and anyone elses simply by being at one of the extremes of the distribution.
So it is not surprising that different authors and testers are sometimes getting different results.

 

[JPeelen]

I of course accept that you cannot report details. But if it is not asking too much, could you give us some orientation along the lines:
a) The drag coefficient (bullet to bullet) of ordinary factory ammunition typically shows a standard deviation of about 0.xx percent.
b) For match ammunition, one may expect a drag coefficient standard deviation of about 0.yy percent.
It would be very helpful to know, what, on the average, can be expected in terms of drag consistency from these types of bullets. Extremes may also be eye openers, but in my view, real world averages are a little more helpful.

P.S. I am delighted your organization uses drag coefficients, not BCs, thereby separating areodynamic qualities from sectional density of a given bullet.

 

[Brians356]

P.S. I am delighted your organization uses drag coefficients, not BCs, thereby separating areodynamic qualities from sectional density of a given bullet.

What organization is that?
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[ballisticdaddy]

Using fixed stands, each gun bullet combination is fired on multiple different occasions with 10 rounds in each serial. The met is measured carefully and monitored with strict maximum values for wind speeds. The bullet is tracked for as long as possible, but for at least 10 seconds.

So you are tracking projectiles for a minimum of 10 seconds which is in the ballpark of 7,000-10,000 yards. I shoot F-class and can tell you from experience that the wind is able to switch many times in less than 1.5 seconds covering 1,000 yards. How are you measuring wind condition / conditions over 7,000 yards for a duration of 10 seconds minimum accurately?

 

[Brians356]

So you are tracking projectiles for a minimum of 10 seconds which is in the ballpark of 7,000-10,000 yards. I shoot F-class and can tell you from experience that the wind is able to switch many times in less than 1.5 seconds covering 1,000 yards. How are you measuring wind condition / conditions over 7,000 yards for a duration of 10 seconds minimum accurately?

If I had to WAG, I'd guess those are artillery shells. And, if so, I'd be loathe to draw too many parallels with miniscule sporting arm projectiles.

Edit: Reading the post again; he said "bullets", and that could not reasonably refer to artillery projectiles, but it could to 50 BMG bullets. So I'll stand aside now, and pop some corn.

 

[ballisticdaddy]

If I had to WAG, I'd guess those are artillery shells. And, if so, I'd be loathe to draw too many parallels with miniscule sporting arm projectiles.

Edit: Reading the post again; he said "bullets", and that could not reasonably refer to artillery projectiles, but it could to 50 BMG bullets. So I'll stand aside now, and pop some corn.

Clearly he was referring to "50 combinations of guns and bullets" and definitely not artillery shells, which is why I asked the question