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Let's argue about BC's

 
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Old 02-15-2011, 05:04 PM
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Let's argue about BC's

Let’s argue about BC’s.

I sent some of our 7mm 200 grain ULD RBBT bullets to Bryan Litz for BC testing. He got .74 – this is a huge difference between the .9 many people use to get their drops to match. For the longest time I’ve been trying to figure out why such a big difference.

At first I thought it might be because Bryan tested these bullets with a 284 at about 2500 fps and a 9 twist barrel possibly causing the bullet to not stabilize for a while which would lower the measured BC. Bryan said that they were stabilizing just fine though.

When I first got my 7mm AM, I zeroed it in at 100, running them at 3250 fps, I plugged in .9 and hit my 1000 yard target about 5” low 3 times. This seemed to confirm the .9 bc. Not long after, I shot at a coyote at 2002 yards and missed left by six inches. Drop looked dead on. I used .9 to get the drop.

When people would email me asking for the BC of this bullet, I’d say “Myself along with many of my customers plug in .9 to get the drops to match. However, Bryan Litz from Berger and Applied Ballistics tested them and got .74 – I’m not sure why such a big difference…” or something like that.

I’ve never had anyone email back and say .9 didn’t work… and no one said .74 worked either. The difference still bugged me (still bugs me). I think there is an honest explanation.

Trying to figure out why, I checked and re-checked everything. I noticed that I left the default scope height of 1.5” on the ballistic calculator (jbm’s) My actual scope height is 2.375 measured from centerline of bore to centerline of scope. After I made that adjustment…. the numbers all changed. I haven’t gone out and tested long range since but the numbers seemed to match with the 5” low hits I was getting at 1000.

Last weekend I went out and shot my most consistent load over a chrony at 10 feet and 888 feet. 3 shots each. The ES of this load runs about 9.

Here’s the data (averages of 3 shots)
Near Velocity: 3300
Far Velocity: 2956
Chrono Separation: 878 ft
Temp: 48
Humidity 28.5 %
Pressure 25.08 (uncorrected)
Altitude 5127

I plugged it into JBM’s “Ballistic Coefficient – Velocity” calculator and got a g7 bc of .740

Why are some people using .9 and hitting targets long range and the measured BC is .74 as per Bryan Litz and myself?I finally think I have a fairly good explanation and I’ll try to explain this later after I’ve shot 1000+ yards with my new numbers.

In the mean time,…. does anyone have any input on this? Why such a big difference? Byan, Kirby, Dick, Len, James, Don, ????? anyone?
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Old 02-15-2011, 05:22 PM
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Re: Let's argue about BC's

Im not having a dig at you, but i have never beleived the stated BC of these bullets to be anywhere near 0.9. There is no such thing as magic bullets...

The reason BC numbers often get mixed up, is that very small details such as zeroing error, aiming error, environmental errors, small velocity errors, and distance errors etc... can very quickly add up to HUGE differences in derived BCs based on drop tests ESPECIALLY at anything less than 1000yds on medium calibers. The way Bryan Litz tests the bullets, using multiple time of flight measurements rather than error prone drop tests, he gets rid of just about all this error and he has the experience to conduct the tests accurately.

most people also beleive that 1000yds is a long way... for a serious long range caliber, it actually isnt far enough. If you want to complete a more accurate drop test for a derived REAL WORLD BC, you really should shoot your bullets out to its maximum supersonic range which can be up near 2000yds if you have a good jacketed bullet in a powerful caliber, or even +2500yds with solids and the cheytac derivative cartridges.

Have you shot to 1 mile with your bullets? I can guarantee if you put 0.9 into your ballistic calculator for 1760yds and your velocity, you will need ALOT more elevation than your calc predicts - provided your bullet is stable thru the transonic region aswell...

Last edited by groper; 02-15-2011 at 05:26 PM.
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Old 02-15-2011, 05:36 PM
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Re: Let's argue about BC's

Paul, I am working a similar issue with the new 338 caliber 185 grain Barnes TTSX. Barnes states a bc of .432 but I am having to plug in closer to .52 to get my drops to match up out to 800 yards. There is two feet of snow at my long range shooting spot and can't work on it until it melts. I am calibrating two chrono's and doing that test when the snow melts. I had my scope height right so it is something else. Let us know what you find out. When I can get on the range and figure mine out I will let you know.
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Old 02-15-2011, 05:43 PM
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Re: Let's argue about BC's

At 3300 FPS the bullets will have a higher G1 BC than they will at 284 velocities. So it isn’t surprising the AM users experience an observed G1 BC higher than the average Bryan got, especially only out to 1000 or so. Had he measured it out of an AM it likely would have gotten a higher G1 number (assuming he measured to the same distances) but the G7 number likely would have been very similar. This is one of the big reason to use G7 BC’s as they should be more nearly constant for all MV’s for bullets of this shape.

I used the JBM calculator with the data you showed and got BC’s of .358 G7 or a .741 G1 which is right in line with what Bryan got. I think you double did the elevation/pressure somehow.

FWIW, don’t feel bad, it’s still an outstanding bullet. It’s really not surprising to me it didn’t quite make an honest .9 as I always kind of thought it wouldn’t. It only has the equivalent SD of a 235 grain 30 cal. It still kicks but on anything else widely available in that caliber (or 30 caliber at the moment for that matter) by a long ways.
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Old 02-15-2011, 05:53 PM
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Re: Let's argue about BC's

If you want to do a more accurate real world BC calculation, rather than do a drop test, provided you own a laptop computer

try this --> http://arxiv.org/ftp/physics/papers/0601/0601102.pdf

Follow the theory outlined in this paper, we are going to use the same theory for BC calculation rather than velocity calculation. We will shoot thru the chrony for the velocity.

Remember to allow 0.001 seconds for every foot your microphone is from the muzzle AND gong.

Make sure you allow for wind speed in your speed of sound, or simply wait for a no wind day.

Make sure you get the air temperature exact.

Shoot a gong at as much range as possible, the mic of your laptop will pic up the sound of a gong @ 1000yds.

Record each shot velocity and time of flight seperately. Once you have enough data, run several calculations to figure BC, and use the median result for your actual real world BC.

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Old 02-15-2011, 06:05 PM
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Re: Let's argue about BC's

I use a G7 BC of 0.370 with JBM and multiple G1s in Exbal (older version) which run from about 0.8 - 0.76 and drop my input velocity. This begins to match my results in the 1800+ yard range.

My feeling is that somehwere in the altitude / calculate pressure input is where the problems arise.
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Old 02-15-2011, 06:26 PM
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Re: Let's argue about BC's

Quote:
Originally Posted by groper View Post
If you want to do a more accurate real world BC calculation, rather than do a drop test, provided you own a laptop computer

try this --> http://arxiv.org/ftp/physics/papers/0601/0601102.pdf
That's genius! Why didn't I think of that? That'll be much easier than the dual chronograph method I've always used. Probably more accurate as well since I can safely do it at a much longer distance.
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