Can you trust advertised ballistics coefficients? Mostly...but not always...

[vancewalker007]

I frequently use LabRadar data (shot tracking files from the SD card) to calculate ballistic coefficients using the JBM Ballistics web tool. Some of the results I got recently for a new rifle prompted me to post this as an FYI.

Here's a table of the 23 different bullets I've tested in the last couple of years, which have sufficient quality data to report, sorted from best to worst in terms of measured vs advertised...

Brand	Caliber	Bullet	Twist	Meas G1	Meas G7	Advert G1	Advert G7	Meas/Adv (%) G1	Meas/Adv (%) G7	n
Hornady	224	62 HPBT	8	0.291		0.274		106%		41
Hornady	308	225 ELDM	9	0.800	0.401	0.777	0.391	103%	103%	20
Hornady	264	143 ELDX	8	0.640	0.313	0.625	0.315	102%	99%	36
Sierra	308	190 SMK	9	0.544	0.271	0.533		102%		5
Hornady	284	175 ELDX	8	0.700	0.343	0.689	0.347	102%	99%	246
Nosler	243	90 AB	10	0.382		0.376		102%		8
Barnes	284	150 TTSX	8	0.454	0.225	0.450		101%		20
Berger	308	215 Hybrid	9	0.690	0.346	0.691	0.354	100%	98%	10
Hornady	308	208 ELDM	9	0.679	0.339	0.690	0.348	98%	97%	12
Hornady	308	208 ELDM	10	0.679	0.340	0.690	0.348	98%	98%	42
Nosler	277	140 AB	10	0.452		0.460		98%		9
Barnes	264	127 LRX	8	0.458	0.224	0.468		98%		16
Barnes	308	168 TTSX	10	0.451	0.226	0.470		96%		8
Barnes	284	145 LRX	8	0.466	0.234	0.486		96%		8
Nosler	277	150 ABLR	10	0.565		0.591		96%		4
Hornady	264	147 ELDM	8	0.661	0.325	0.697	0.351	95%	93%	93
Barnes	284	139 LRX	8	0.445	0.223	0.470		95%		9
Badlands	308	195 BDII	9	0.625	0.311	0.675	0.345	93%	90%	19
Badlands	284	140 BDII	8	0.516	0.259	0.560	0.287	92%	90%	18
Barnes	264	145 MB	8	0.647	0.321	0.703	0.350	92%	92%	11
Hammer	284	143 HH	8	0.399	0.200		0.230		87%	3
Hammer	284	140 AH	8	0.389	0.194		0.225		86%	5
Hammer	264	124 HH	8	0.413	0.202		0.245		82%	7

This data is all from tracking at least 100 yards, and uses atmospheric data from a Kestrel 3500 and my iPhone/Watch barometer. All bullets were fired from barrels with twist rates that were at or faster than recommended.

17 of 23 bullets were within 5% of quoted (18 if you count the 62 HPBT that was 6% better than quoted).
7 of 23 were better than quoted.
3 of 7 manufacturers had bullets that were consistently lower than quoted
2 of 7 manufacturers had no bullets within 7% of quoted
1 of 7 manufacturers had no bullets within 10% of quoted.

I may be a bit cynical, but I can't get past the feeling that some manufacturers are either incompetent or intentionally misleading when they provide a B.C. value that is so far off of reality. If I can get values within spitting distance of Berger/Hornady/Sierra/Nosler with a $600 consumer grade chronograph, a $150 weather meter, and a free online calculator, what possible excuse is there for advertising inaccurate B.C. values when you make bullets for a living?

For those that will say "Never Believe Anything! Validate with drops! Who cares about B.C.!" that's just silly.

We all use manufacturer provided data to make informed choices every day, and pretending otherwise is disingenuous. Would you shop at a gas station that required you buy the gas first, then confirm for yourself that they weren't shorting you 10% of the gas you thought you received?

Their BCs are calculated using their unique tools, barrels, meters, measure point, atmosphere etc. These all have their own built-in error/accuracy levels. Unless you were using the exact tools they were using to compute their BCs you will always be a little different. Because of this, the BC is a starting point and cannot be relied on as an absolute value, just like a max powder load in a loading manual. It looks like most of your calculations were +6%/-8% for the G1s and a little worse for the G7s but still in the ballpark which would allow someone to quickly refine their dope for various distances. Maybe contact some of the manufactures and get the exact process they used to determine their numbers and run it again using their approach.

 

[Pdvdh]

Their BCs are calculated using their unique tools, barrels, meters, measure point, atmosphere etc. These all have their own built-in error/accuracy levels. Unless you were using the exact tools they were using to compute their BCs you will always be a little different. Because of this, the BC is a starting point and cannot be relied on as an absolute value, just like a max powder load in a loading manual. It looks like most of your calculations were +6%/-8% for the G1s and a little worse for the G7s but still in the ballpark which would allow someone to quickly refine their dope for various distances. Maybe contact some of the manufactures and get the exact process they used to determine their numbers and run it again using their approach.

The BIG picture:
If manufacturers are not using velocity decay or time of flight (which is a direct result of bullet velocity), they've compromised with their method of BC determination. Which is how Bryan Litz exposed manufacturers' inflated BC values. Litz collected "velocity/time of flight" based BC values for various manufacturer's bullets. It eliminates common errors associated with BC values determined by down-range bullet drops. If Bryan Litz hadn't generated velocity-based bullet BC values, a number of bullet manufacturers would no doubt still be grossly exaggerating BC values for profit.

Yes, there are some minor nuances in the equipment used to collect BC value determination that can result in minor error. Specific barrels and barrel twist rates are two examples. These minor influences on bullet BC value determination will never be eliminated. But their affect is very minor, IMO.

All bullet manufacturers should identify the standard atmospheric conditions their advertised BC values have been normalized to. That should never be a factor causing discrepancy in advertised versus true BC values.

 

[del2les]

While I agree some BC's stated are more marketing than actual, was much more true before the advent of inexpensive chronographs, real inflight BC's change based on many factors.

 

[ARlife4me]

Nosler keeps changing their averages or their bullets are degrading, changing location/altitude or time of year tested! They come out with a new bullet of the same weight per caliber and the older 1 has a bc drop. Probably shouldn't matter, but it's irritating! As long as I/you know needed drop/drift info then advertised bc is almost meaningless. 1 day I'll check at my location and temp I shoot in! If I ever take the time to shoot past 600 yards it might make a difference.

 

[Taylorbok]

BC also varies from barrel to barrel. Hornady 4DOF uses something called Axial form factor which essentially alters the BC you input to match your impact, they cap this number at .90 and 1.10 which is 10% either direction so basically in your testing everything was within tolerance and that's just the BC your rifles produce with those bullets. Even the same bullet out of the same barrel with the same load will have BC variation from round to round, Applied Ballistic actually uses this to monitor barrel life, when BC variation according to their radar get's bad they know the barrel is toast.

 

[del2les]

Rough bullets fired from worn barrels have lower BC's, and it was one reason why moly bullets would strike higher at 1k than many of our non moly counterparts. The engraving was smoother and reduced drag in flight.

 

[akmtnhnt]

Rough bullets fired from worn barrels have lower BC's, and it was one reason why moly bullets would strike higher at 1k than many of our non moly counterparts. The engraving was smoother and reduced drag in flight.

That's always been the thing that modeled(estimated, not actually shot) BC doesn't seem to be accounting for. Engraving. The rifling marks on a bullet create a crap ton of drag. I would imagine there's a measurable difference in drag on a bullet depending on the bullet's shape with respect to how much of the bullet gets engraved, and the type and number of lands/grooves.

 

[entoptics]

Sigh...

I've already addressed "normal variation", but for those that require further clarification...

0) My data set includes 23 bullets, 11 different barrels, and in some bullets, dozens of measurements in different weather conditions. That should cover quite a bit of "normal variation".

1) My setups had either recommended or faster twist rates. My velocity window is at the high end. The vast majority of the "problematic" examples were fired during load development in newish 5R custom barrels. Those 3 parameters should skew my data towards HIGHER than "normal" B.C. values if anything.

2) "Normal variation" from honest uncertainty in measurement (both mine and the manufacturers) would give normally distributed values both above and below the stated B.C. That is obviously not the case. 15 out of 23 bullets were below quoted. only 2 were higher by >2%, yet 11 were ≥ 4% low.

3) "Normal variation" from honest uncertainty wouldn't appear to be manufacturer dependent...

Lastly, and most importantly, this could all be mitigated if manufacturers agreed on a somewhat standardized testing procedure.

If only there was a simple method, like using some sort of consumer grade radar, a weather meter, and a free online calculator...Nah, that's just ridiculous, what manufacturers could possibly have the time, skill, and extreme resources to take on such a monumental task...

I'm just glad that everyone "knows" that B.C. is extremely variable and nearly impossible to determine reliably. Otherwise some might conclude that certain manufacturers are benefiting from unwarranted confusion (or willful ignorance)...

 

[del2les]

Sigh...

I've already addressed "normal variation", but for those that require further clarification...

0) My data set includes 23 bullets, 11 different barrels, and in some bullets, dozens of measurements in different weather conditions. That should cover quite a bit of "normal variation".

1) My setups had either recommended or faster twist rates. My velocity window is at the high end. The vast majority of the "problematic" examples were fired during load development in newish 5R custom barrels. Those 3 parameters should skew my data towards HIGHER than "normal" B.C. values if anything.

2) "Normal variation" from honest uncertainty in measurement (both mine and the manufacturers) would give normally distributed values both above and below the stated B.C. That is obviously not the case. 15 out of 23 bullets were below quoted. only 2 were higher by >2%, yet 11 were ≥ 4% low.

3) "Normal variation" from honest uncertainty wouldn't appear to be manufacturer dependent...

Lastly, and most importantly, this could all be mitigated if manufacturers agreed on a somewhat standardized testing procedure.

If only there was a simple method, like using some sort of consumer grade radar, a weather meter, and a free online calculator...Nah, that's just ridiculous, what manufacturers could possibly have the time, skill, and extreme resources to take on such a monumental task...

I'm just glad that everyone "knows" that B.C. is extremely variable and nearly impossible to determine reliably. Otherwise some might conclude that certain manufacturers are benefiting from unwarranted confusion (or willful ignorance)...

And the search for perfection continues

Chaos Theory cannot win. Thanks for sharing.

 

[Taylorbok]

It would have been interesting to see how your numbers would have compared if you would have shot them on target and also calculated on drop.

Berger shoots their bullets on Radar so theirs are measured and Hornady has started doing the same. I was at NF ELR and AB had their Radar, there was lots of people with BC higher and lower than what they were expecting but barely worth talking about. once you get trued the variation is the bigger worry.

 

[entoptics]

It would have been interesting to see how your numbers would have compared if you would have shot them on target and also calculated on drop.

Berger shoots their bullets on Radar so theirs are measured and Hornady has started doing the same. I was at NF ELR and AB had their Radar, there was lots of people with BC higher and lower than what they were expecting but barely worth talking about. once you get trued the variation is the bigger worry.

I've fired dozens of ELDX and ELDM on target while recording with LabRadar. As my data suggests, those bullets land pretty dang close to where the manufacturer's B.C. predicts they should at longer range. On Sunday, I zeroed the 225 ELDM at 184 yards, then plugged my calculated B.C. into Ballistic, and fired 5 rounds at 625 yards. The center of the 3/4 MOA group was less than 1/4 MOA off vertically. That's better than I can shoot, so take that with a grain of salt.

The reason I started routinely doing the calculations during load development in the first place, was because some bullets did not land even close to where the manufacturers claimed they would (hint: the problem bullets are in the data table and aren't Berger, Hornady, or Sierra)...

 

[Mikecr]

I wish bullet makers would determine and offer drag curves for their bullets, and let us determine local BC.
Of course ballistic software would need to allow custom drag curve use.
With this, BC (whatever determined to be) would remain a constant downrange.
Without this, I use a modified Pejsa method to adjust velocity decay rate to calibrate field results.

As it stands, a 100yd G1BC would be different than it's 500yd G1BC, because our bullets do not match the G1 form.
That G7BC can vary way less just shows how important the drag curve is to nailing this.

 

[Bob Wright]

As Ronald Reagan once stated: Trust but verify.
Oddly enough, one of the bullets you have as one of the worst advertised/measured BC values, I had a very accurate BC. The variables were 7200 ft elevation prior to an 8300 elevation hunt, and about 40° expected change in temp when hunting. I shot at 600 yards to measure drops and group sizes. It was so close to the stated BC I shrugged when I made my ballistic app changes as almost unnecessary.
I certainly have had other stated BC in another manufacturer simply fall apart when doing the same exercise. Those bullets are essentially junk.
Verify is all I can do.

 

[MagnumManiac]

The reason for my post was such that nobody can rely on what's printed on the box as gospel.
For instance, when developing my ELR wildcats, I wanted 3200fps minimum with 300g bullets, at the time, in 338, there was 1 choice that wasn't a custom, so I had my own design cut from brass, it wasn't for hunting, so it didn't really matter, then it changed to a guilding metal for better weight, the end result is a 323g bullet.
Anyway, the only way to determine BC was using drops, and to my surprise it was way beyond what I predicted. Now, if this is normally how it's done to verify, I know that my extra velocity than normal cartridges increased BC, then factory bullets would have the same affect, no?

Cheers.