Range Report Testing for ballistic coefficients

[dbooksta]

What's the standard practice for getting ballistic coefficients?

My understanding is that I can't just use a bullet manufacturer's reported BC -- if I'm using different rifles, barrel lengths, and suppressors don't I need to actually test each for BC?

And since I don't have easy access to ranges longer than 100 yards do I need to use two chronographs (one near muzzle, one at the 100-yard line) to get the BC? Or are there other tricks?

 

[Lindy]

Re: Testing for ballistic coefficients

Easiest way is to start with the manufacturer's coefficient, shoot it to long range - the longer the better, as long as the bullet is still supersonic - and then adjust the B.C. until your shooting result matches the ballistic program output for that range. That assumes you have access to a chronograph, so you know the muzzle velocity.

 

[BryanLitz]

Re: Testing for ballistic coefficients

dbooksta,
Bullets have very consistent BC's when fired from different rifles, provided the barrels are properly stabilizing the bullets. As long as your barrel has at least the recommended twist rate, the bullets will fly the same and have the same BC from any rifle.
A G1 BC will be velocity dependent but if you use BC's referenced to the G7 standard you don't have to worry about velocity dependence.
It is possible to derive a BC from drop data as Lindy suggested but there are many variables that have to be measured exactly in order to get an accurate result. No offense to Lindy or anyone who's done this, but very rarely does it result in BC's within 10% of the real value. Part of the challenge is determining the center of your group at long range. If you shoot a 15" group at 1000 yards then there's at least some uncertainty in the <span style="font-style: italic">actual</span> drop of your bullets at that range which corresponds to uncertainty in your derived BC.
Using two chronographs is possible, but tricky. First you need to calibrate them so you know they're in sync. You need them separated by at least 100 yards for the test to mitigate error; more separation is better. The farther you separate them, the more likely it is that you will hit the one downrange.
The method I use to measure BC's is based on time of flight measurements using acoustic sensors in multiple segments over 600 yards. The process is repeatable withing +/- 1%, but everything has to be measured precisely.
I'm not discouraging you from trying to measure BC, but wanted you to be informed and realistic about the challenges and attention to detail that's required to get meaningful results.
Here's a link to a paper that describes a well done test where BC was derived by drop for several bullets:
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=966431#Post966431
click on the 'my report' link on the first post of this thread.

If you have particular bullets in mind, I may have data for them as I've tested many bullets for BC.

Take care,
-Bryan

 

[Lindy]

Re: Testing for ballistic coefficients

No offense taken, Bryan.

And I'm not trying to actually <span style="font-style: italic">measure</span> the B.C. - I just want the ballistic software to produce a result which agrees with my shooting data. That removes a bunch of variables.

 

[kyreloader]

Re: Testing for ballistic coefficients

Bryan, what about the 140g Berger VLDs 6.5mms compared to the new 140.5g Berger Target Match bullets that are coming soon(midway usa).

Can you include both G1 and G7 numbers?

I am wanting to try out the new 140.5s.

 

[BryanLitz]

Re: Testing for ballistic coefficients

Kyreloader,
Here are the BC's for the bullets you asked about:
Berger 6.5mm 140 VLD
G1 BC = 0.595 lb/in^2 (average from 3000 fps to 1500 fps)
G7 BC = 0.304 lb/in^2 (valid for all speeds)

Berger 6.5mm 140.5 LRBT
G1 BC = 0.592 lb/in^2 (average from 3000 fps to 1500 fps)
G7 BC = 0.303 lb/in^2 (valid for all speeds)

As you can see, the BC's are nearly identical for the two bullets. You might find the LRBT's to be more length tolerant than the VLD's. They haven't been tested extensively enough to know if there is an inherent accuracy difference compared to the VLD's (other than the sensitivity to seating depth).

-Bryan

 

[kyreloader]

Re: Testing for ballistic coefficients

Sorry to keep asking, but Lapua still lists their 139g Scenar's BC over .600. Comparing your 140g VLD's to their bullets, I have trouble believing the published BC.

Have you tested that bullet, if so, is it close to published? If you dont want to post, please PM me if you get a minute.

Thanks in advance.

 

[BryanLitz]

Re: Testing for ballistic coefficients

Kyle,

Here are my test results for the Lapua 139 Scenar:
G1 BC = 0.557 lb/in^2 (average from 3000 fps to 1500 fps)
G7 BC = 0.285 lb/in^2 (valid for all speeds)

The Lapua's I tested have larger than average tip diameters (0.076"). 139 Scenars with more average tip diameters like 0.060" or 0.065" will have higher BC's by about ~2-3%.

At 3000 fps, the Scenar has a BC very close to 0.600, and it falls to ~0.500 at 1500 fps. Lapua has all that good radar data for their bullets, but in some cases, the BC's they advertise are not consistent with their own raw drag data. I don't know why, maybe they just don't want to change what they've been advertising for so long.

I don't want this to come across as a negative criticism about Lapua. They make very good bullets.

-Bryan

 

[Ratbert]

Re: Testing for ballistic coefficients

I can't express how much I appreciate that:

1)Berger now publishes usable BC info
2)You take the time to test non-Berger bullets and share that info as well.

Your G7 figures for the 155 Scenar have been quite useful to me. So much so that I decided I'm running Bergers in my 6.5 when it's complete.

Just keep this in your pocket any time a bean-counter asks you to justify your job.

 

[dbooksta]

Re: Testing for ballistic coefficients

Bryan,

Thank you for the details. I didn't realize that BC was so dependent on the ballistic model I'm going to use!

One question regarding your explanation: You say that if the bullet is "properly stabilized" the ballistic model is valid. But what's the margin of error for stabilization? Presumably there's exactly one twist rate that for a given load perfectly stabilizes it, and any deviation in twist rate or powder from those perfect numbers will either over- or under-stabilize the bullet -- which means that the BC would have to be changed, right? Or are does an over-stabilized bullet not have an appreciably different BC within small-caliber distances?

The thing that prompted this question was an article in a recent magazine (probably Guns, Guns & Ammo, or American Rifleman -- of course I can't find it now!) in which the author reported getting an impossibly high BC (like .8) out of one rifle with one particular load. But it was repeatable so he and his consultants just concluded that his rifle was doing an extra-good job of stabilizing that load.

 

[BryanLitz]

Re: Testing for ballistic coefficients

Ratbert,
Thanks for your comments. Fortunately we're all on the same page at Berger as far as doing what it takes to enhance the shooting experience, regardless of brand.

DB,
Actually the window of <span style="font-style: italic">proper</span> stability is quite large. On the low end, you have to use at least the minimum recommended twist. On the high end, well, most of the adverse affects cited for <span style="font-style: italic">over stability</span> are myths. The most adverse effect of very high stability levels is extra spin drift. Other than that, elevation is not affected and BC is not affected.
I think most of the myths about over stability stem from imperfect understanding of the sights including: imperfect zero, click adjustments not being true, canted reticle, etc.
I occasionally see cases of extremely high (and low) BC's being reported by those conducting tests. You'll find that in most of those cases, the non-physical results are caused by oversights in the test.
It's a comfort to know that most of the smoke and mirrors surrounding BC is just that; smoke and mirrors. At the end of the day, actual BC's are very well behaved and constant from all rifles across a large range of stability levels. If two bullets have the same shape and one is heavier, the heavier one has a higher BC. If two bullets are the same weight, and one is more streamlined, the more streamlined one has a higher BC. This is common sense but many 'advertised' numbers contradict these basic trends and it causes unnecessary confusion.

Take care and good shooting,
-Bryan

 

[Jonathan]

Re: Testing for ballistic coefficients

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">On the high end, well, most of the adverse affects cited for <span style="font-style: italic">over stability</span> are myths. The most adverse effect of very high stability levels is extra spin drift. Other than that, elevation is not affected and BC is not affected.</div></div>

Out of curiosity, what are the effects that people claim for overstabilization?

 

[BryanLitz]

Re: Testing for ballistic coefficients

The most common misconception is that a bullet that's spun <span style="font-style: italic">too fast</span> will fail to trace with the trajectory. In other words, it will fly point high on the downrange leg of the trajectory. Not true. At least not in a direct fire application. A bullet may fail to trace if you're shooting it at a very high angle, like near 90 degrees straight up and you expect it to come back down point first, but that's not a practical situation. For normal angles of fire (less than a couple degrees), bullets have no trouble tracing with the trajectory. The fear is that the nose high flight would cause the bullet to present a greater frontal area to the oncoming air flow and cause an effective decrease in the BC.

 

[jwp475]

Re: Testing for ballistic coefficients

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">The process is repeatable withing +/- 1%, but everything has to be measured precisely. </div></div>

That is incredible. + or - 1%. I have seen the same rifle and load vary the BC from shot to shot more than that with Olher equipment useing Acoustic targets. What type of system are you using?

 

[straycat40]

Re: Testing for ballistic coefficients

Hi Bryan,

Do you have any BC data for the .308 168gr A-MAX?

Thanks

 

[BryanLitz]

Re: Testing for ballistic coefficients

Chris,
The BC's for the .30 cal 168 grain Amax are:
G1 BC: 0.450 lb/in^2 (average from 3000 fps to 1500 fps)
G7 BC: 0.230 lb/in^2 (valid for all speeds)

jwp,
The +/- 1% refers to the average BC that's determined for a specific bullet type. For example, 5 shots may produce BC's that are +/- 2% or 3% for each shot, but the average BC for the 5 shots is withing +/- 1% of the average BC measured for 5 shots of that bullet on a different day.

The measured shot-to-shot variation in BC is partially a resolution issue with the measuring equipment, and partially real; meaning bullets from the same box can have very minor differences in their BC's due to irregularities on the meplats, etc. My equipment and procedure is designed to determine the average BC of a certain bullet type with as much accuracy as possible, not necessarily to determine shot to shot variations in BC.

My test equipment uses acoustic sensors and wireless transmitters placed in multiple segments. I usually use 600 yards unless the bullets dictate otherwise (you can't keep a 30 grain 22 cal bullet supersonic for 600 yards). The result is a sound file that contains the multiple 'cracks' of the bullet as it passes each sensor. The exact time of the bullets passage can be determined within 0.0005 seconds, which means that if the distances between the sensors, the muzzle velocity, the atmospherics, and other details of the test set up are measured carefully, BC's can be determined from the time of flight data very precisely.

The equipment is not commercial, I've had to assemble most of it myself because the existing systems (Oehlers) use cables for the sensors and allow testing at only one range at a time. Using wireless transmitters increases the range that the test can be conducted over and you can capture time of flight in multiple segments.

If anyone is interested in one of the sound files of a bullet passing the mics, email me at: [email protected] and I'll send you a file. Unless you have some high end audio processing software you won't be able to determine exact times of flight, but it's still cool to hear.

-Bryan

 

[jwp475]

Re: Testing for ballistic coefficients



Thanks

 

[dbooksta]

Re: Testing for ballistic coefficients

Bryan, I called both Sierra (I shoot their .308 168gr BTHPM) and Remington (.223 68gr BTHPM) and neither had a clue about G7 model BC's. Is it sufficient to recompute the G7 BC based on the velocities indicated by their G1 models, or is accuracy lost in that process versus direct testing?

 

[BryanLitz]

Re: Testing for ballistic coefficients

D,
The ballisticians at Sierra know about G7 BC's, even if the guys on the 1-800 tech line don't. Here's a link to a paper written by Sierras ballisticians Bill McDonald and Ted Algrem:
http://www.exteriorballistics.com/ebexplained/articles/the_ballistic_coefficient.pdf

At the very end of the article it talks about the other G functions and acknowledges the better fit of modern bullets to the G7 standard.

To answer your question about converting from G1 to G7; it is possible, HOWEVER, you have to know the exact speed that the G1 BC is valid for, and it has to be accurate to start with. The bottom line is that if the BC wasn't established by careful test firing, it's not guaranteed to be accurate (G1 or G7).

Here's a link to one of JBM's utilities that does the conversion, but remember you have to know the velocity that the original BC is valid for in order for the conversion to be valid.

http://www.eskimo.com/~jbm/cgi-bin/jbmgf-5.0.cgi

-Bryan

 

[Lindy]

Re: Testing for ballistic coefficients

And Sierra's web site generally publishes more than one velocity range, at least for the SMKs, with the corresponding B.C. for each range. Nice of them, I think.

 

[BryanLitz]

Re: Testing for ballistic coefficients

Publishing velocity banded G1 BC's is certainly better than giving a single BC. You can get quite an accurate trajectory with the velocity banded BC's (if your program allows the input of multiple BC's). The need for multiple BC's is a result of modern bullets being a poor match to the G1 standard. If G7 were used, there would be no need for multiple BC's. If you did describe a G7 BC in velocity bands, there would only be about 0.001 or 0.002 difference in the G7 BC in the different bands.
-Bryan

 

[jthyttin]

Re: Testing for ballistic coefficients

Bryan, how do you keep time sync on your measuring equipment? Do you have different clocks on each sensor (if so, how do you keep them in sync?) or how do you enforce realtime requirement on networking level (between sensor and receiving end)?

And, you don't happen to have G7 BC for Lapua 6.5mm 123gr Scenar?

 

[jwp475]

Re: Testing for ballistic coefficients



Brain, do you have numbers for the 300 Grain SMK in 338 cal.

 

[BryanLitz]

Re: Testing for ballistic coefficients

JT,
Good question. It was a major challenge of the design. All of the sensors continuously transmit what they 'hear' to one central recorder. The signals are received and combined and recorded on the same device against the same clock, so synchronization is enforced.
I haven't tested the 123 Lapua Scenar. However, if it has the same ogive and boat tail as the 108 and 139 (which I have tested), it should have a G7 BC very close to 0.255.

JWP,
I don't have a .338 cal test barrel yet. It's being made now and I should have data on .338's by this summer.

-Bryan