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chart probems

 
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  #1  
Old 09-29-2009, 11:33 PM
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chart probems

I was testing a new load today and with all the correct input data I am close enough to be just human error out to 800 yards. At that point things change fast, at 1125 I am 31.2 moa and my chart shows 33.4 near as I can tell I shoot about 24 inches high if I use the chart. I am using a jbm chart and again ALL of my input data is correct. any input?
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  #2  
Old 09-30-2009, 06:55 AM
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Re: chart probems

Bud,
I'd like to take a shot at helping you figure this out. It will take a lot more information. My first questions are:
1) What bullet and BC are you using?
2) What muzzle velocity and how did you measure it?
3) How are you measuring range?
4) Have you verified your scope adjustment, ie, does 1 MOA of adjustment really move the crosshairs exactly 1 MOA?
5) Have you re-verified the zero after observing the mis-match at long range?
6) What ranges did you shoot and what were the drops at each range?
7) Was there any terrain that could cause vertical wind currents?

Everything has an explanation; the question is; can we figure it out?

-Bryan
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Author of: Applied Ballistics for Long Range Shooting
And: Accuracy and Precision for Long Range Shooting

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  #3  
Old 09-30-2009, 09:08 AM
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Re: chart probems

Here are the specifics of the load, it is a 264 win mag with a 140smk, velocity is 2930 shot thru a crony at 10 feet. sight height is 1.68 inches. scope is a mark 4, rangefinder is a swaro, gun is still zero at 100 and the shooting elevation is 3500 feet. I use the standard pressure and temperature for the program. there is no wind or drafts to speak of between the rifle and target and I have had the same results with this new load for the last 3 days. all of our rangefinders measure the distance within a yard or two. my crony could be off I suppose but it seems to be correct with all of my other rifles and the jbm charts.
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  #4  
Old 09-30-2009, 09:40 AM
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Re: chart probems

Bud,
I think the problem may come from using the advertised G1 BC(s).
I haven't measured the G7 BC for the 140 MK (I do have the 142 MK).
Based on Sierra's advertised G1 BCs, this bullet should have a G7 BC of 0.270*.

I've generated the table below using that G7 BC and the following inputs;

MV = 2935 fps
I added 5 fps for velocity lost in the 10' from muzzle to target. This isn't a big deal considering there's probably more error than that in the Chrony.

Temp = 59 deg F
Pressure = 26.33 inHg (standard pressure at 3500 ft)
Sight Height = 1.68"
Zero range = 100 yards

Note that the bullet path in the table is -375.87" at 1125 yards.
That's equal to 31.9 MOA. You said you observed 31.2 MOA at that range, a difference of 0.7 MOA.

You said your chart showed more drop; 33.4 MOA at that range. What BC did you use to generate that chart?

How does the bullet path in the chart below compare to your records at other ranges?

Range Velocity Trajectory TOF Drift
(yards) (fps) (inches) (sec) (inches)
0 2935 -1.68 0.0000 0.00
25 2896 -0.86 0.0257 0.00
50 2857 -0.31 0.0518 0.00
75 2818 -0.02 0.0782 0.00
100 2780 -0.00 0.1050 0.00
125 2741 -0.26 0.1322 0.00
150 2704 -0.82 0.1597 0.00
175 2666 -1.67 0.1877 0.00
200 2629 -2.82 0.2160 0.00
225 2592 -4.29 0.2447 0.00
250 2555 -6.09 0.2739 0.00
275 2518 -8.21 0.3034 0.00
300 2482 -10.68 0.3334 0.00
325 2446 -13.50 0.3639 0.00
350 2411 -16.68 0.3947 0.00
375 2375 -20.24 0.4261 0.00
400 2340 -24.18 0.4579 0.00
425 2306 -28.52 0.4902 0.00
450 2271 -33.27 0.5229 0.00
475 2237 -38.44 0.5562 0.00
500 2203 -44.04 0.5900 0.00
525 2170 -50.09 0.6243 0.00
550 2137 -56.60 0.6591 0.00
575 2104 -63.59 0.6945 0.00
600 2071 -71.07 0.7304 0.00
625 2039 -79.05 0.7669 0.00
650 2006 -87.56 0.8040 0.00
675 1975 -96.60 0.8417 0.00
700 1943 -106.21 0.8800 0.00
725 1912 -116.38 0.9189 0.00
750 1881 -127.16 0.9584 0.00
775 1850 -138.54 0.9986 0.00
800 1819 -150.57 1.0395 0.00
825 1789 -163.24 1.0811 0.00
850 1759 -176.60 1.1234 0.00
875 1729 -190.66 1.1664 0.00
900 1699 -205.45 1.2102 0.00
925 1670 -220.98 1.2547 0.00
950 1640 -237.30 1.3000 0.00
975 1611 -254.43 1.3461 0.00
1000 1583 -272.39 1.3931 0.00
1025 1554 -291.22 1.4409 0.00
1050 1526 -310.95 1.4896 0.00
1075 1498 -331.61 1.5392 0.00
1100 1470 -353.24 1.5898 0.00
1125 1442 -375.87 1.6413 0.00
1150 1415 -399.55 1.6938 0.00
1175 1388 -424.32 1.7474 0.00
1200 1361 -450.22 1.8019 0.00
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Bryan Litz
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And: Accuracy and Precision for Long Range Shooting


Last edited by BryanLitz; 09-30-2009 at 10:01 AM.
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  #5  
Old 09-30-2009, 11:04 AM
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Re: chart probems

Brian, I agree that the bc may be off in the bullets what I have a hard time believing is the bc is higher than they advertise. it would be my guess that if a bullet manufacturer was to fib a little it would be to the good and not the other way around. the chart you produced is also very close out to around 800 yards within a couple inches or so, it is actually only 2 inches different than mine at 800 and I am not good enough to tell 2 inches at 800. I can sure tell 2 moa at 1125 though. I have shot at 100,200,400,500,800,1000,and 1125 which is as far as I can shoot where I do and is farther that I have energy to shoot deer anyway. the chart is very close out to 800 close enough that I attribute it to drinking to much coffee before shooting but 1000 is way more than that and 1125 is close to 2 feet and better that it should be. I used the g1 bc to generate the current chart by the way. I have NOT tested the crony against another but it seems very accurate with my other guns and loads using jbm charts. the loads I have with your vld's are right on the money with this gun, crony and jbm stats.
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  #6  
Old 09-30-2009, 01:23 PM
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Re: chart probems

Quote:
Brian, I agree that the bc may be off in the bullets what I have a hard time believing is the bc is higher than they advertise. it would be my guess that if a bullet manufacturer was to fib a little it would be to the good and not the other way around.
A couple comments here.

1) Sierra test fires their bullets for BC. Of all the companies bullets' I've measured, Sierra, on average, has the least amount of error in their BC's. Some of their BC's are higher than mine, some are lower, as much as +/- 5% is normal, on rare occasions there can be up to 10% or more error between our measurements. In this case of the 6.5mm 140 grain MK, the BC would only have to be in error by about -4% or -5% to see the discrepancy in path that you observe at 1125 yards.

2) Advertised BC's are corrected for standard atmospheric conditions. The problem is, there are a couple definitions of 'standard atmosphere'. Sierra's advertised BC's are corrected to the older Army Standard Metro (ASM) atmosphere model. JBM uses a more modern atmosphere model; the International Civil Aviation Organization (ICAO). When you use a BC that's corrected for one standard in a program that uses a different standard, you get some error. In this case (using a Sierra BC in JBM), you induce about a -2% error in the effective BC. In other words, JBM is interpreting Sierra's BCs as being 2% lower than Sierra intends for them to be represented.

3) When Sierra tests a new bullet for BC, the BC they advertise hardly ever changes. They'll advertise the same BC that they measured for a bullet 2 decades ago. They may be on their 100th set of bullet dies and the bullet's shape may be quite different than the original samples that were tested, but I've never known Sierra to re-test and update their advertised BC's. This is another possible reason why their advertised BC's may 'drift' a little from actual values.

The point is, I wouldn't necessarily assume that when a companies advertised BC's are off that they're always high; sometimes the error is that they're low. Also, the error isn't always intentional.

Measuring BC's isn't very hard.

Measuring BC's accurately is very hard and requires extreme attention to detail.

Take care,
-Bryan
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Bryan Litz
Ballistician

Author of: Applied Ballistics for Long Range Shooting
And: Accuracy and Precision for Long Range Shooting

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  #7  
Old 09-30-2009, 03:52 PM
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Posts: 161
Re: chart probems

Well it appears that to me that the bc must be higher than published, that would have been my last guess however. if I add 5-6% to the bc it gets me closer than anything so I'll use that and run with it I guess. thanks for the input and info.
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