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Help calculating a BC from two speeds

 
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  #1  
Old 10-02-2007, 08:02 AM
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Join Date: Oct 2007
Posts: 156
Help calculating a BC from two speeds

OK, I guess I can't get it figured out on my own. I have good data, but can't seem to figure out how to apply it to get consistant results. I am working with a new bullet (Wildcat ULD RBBT .277cal 150grn) and get a variety of results on a calculated B.C. From the data below I have values ranging from .516 to .590 (I think these are all G1 drag models) depending on what program I use, and what "atmospheric" boxes I check. If someone who understands the weather stuff better than me could help me out I would greatly appreciate it. Here is the data.

Chronograph was placed at 10ft & 198 yards (195 yards apart)
Initial speed average 3095fps.
End speed average 2775fps.
It was 60-61 degrees, 33% relative humidity, elevation was 3680ft.
Noaa reported the barometer at 29.85 or 1011.3mb. The station is at 3398ft elevation.
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  #2  
Old 10-02-2007, 12:15 PM
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Join Date: Jan 2004
Location: Blackfoot, Idaho
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I appreciate your quandary. Replies to your post should be quite revealing. We'll learn tons from this one.

I used two of the more popular computer codes to give it a go (not Exbal) and got zero confidence scattergun results. I see why I'm more comfortable using drops at distances.

OK here goes: (Hope I don't get beatup too badly;))

RSI: Sea level bc = 0.346
At elevation bc = 0.350

PointBlank: Had to set the MV to 3130 to get 3095 @ 10yds.

BC = 0.238

I'm thinking I could 'guess' the bc closer than this result

Am REALLY looking forward to other's results.

Thanks for the opportunity.
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Last edited by royinidaho; 10-02-2007 at 12:18 PM.
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  #3  
Old 10-02-2007, 01:18 PM
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Location: Mukilteo, WA
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HERE is a good place to do it. I'm getting about .523. Pretty darn good for a 150 gr 270.
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  #4  
Old 10-02-2007, 01:45 PM
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Join Date: Jan 2002
Location: Wenatchee, WA
Posts: 804
I haven't messed around finding B.C.s from velocity myself, but here are some general thoughts

You need to know if your atmo pressure from NOAA is corrected or not.

The NOAA data for my local airport (KEAT, ~1280' ASL) is available as both corrected (29.90" Hg this morning), or raw ('station') pressure (28.58" Hg). The NOAA data for Raton (6350' ASL), as an example, is only available as corrected pressure (30.13" Hg). The actual uncorrected pressure at the range there (6600' ASL) is more like 23.7" Hg, which is a huge difference.

If you can get the raw uncorrected station pressure, then you need to uncheck the box 'Corrected pressure' in JBM and Exbal. Otherwise it takes that pressure and *then* corrects it for the altitude.

If all you can get is the corrected pressure, and the altitude, then enter them and make sure the box for 'Corrected pressure' *is* checked. It may not be quite as precise as having your own Kestrel or similar weather meter on site to take actual raw pressures, but it will likely be close.

HTH,

Monte
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  #5  
Old 10-02-2007, 02:17 PM
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Join Date: Dec 2001
Location: Mukilteo, WA
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All very true. But, 29.85 at 3700 ft altitude? I'm going to go out on a limb and say that was corrected. ;)

Though I now realize I was using the wrong altitude. :( More like .516 at 3700.
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  #6  
Old 10-02-2007, 03:40 PM
Platinum Member
 
Join Date: Jan 2004
Location: Blackfoot, Idaho
Posts: 8,047
I appreciate your quandary. Replies to your post should be quite revealing. I'm betting we'll learn tons from the replies.

I used two of the more popular computer codes to give it a go (not Exbal) and go scattergun results. I see why I'm more comfortable using drops at distances.

OK here goes: (Hope I don't get beatup too badly;))

RSI: Sea level bc = 0.346
At elevation bc = 0.350

PointBlank: Had to set the MV to 3130 to get 3095 @ 10yds.

BC = 0.238

I'm thinking I could 'guess' the bc closer than these results

Am REALLY looking forward to other's results.

Thanks for the opportunity.
__________________
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!
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  #7  
Old 10-02-2007, 05:15 PM
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Join Date: Oct 2007
Posts: 156
Thanks for the replys guys. Royinidaho. I might not have made it clear, my starting position for the chronograph was at about 10 feet not yards. this might change the results you get.

Jon, I was guessing that it was a corrected pressure as well, but was still not quite sure how to apply it. I am thinking your 5.16-5.23 is right. I couldn't believe that the .575-.585 numbers I was getting could be right.

milanuk, you helped me clear up the corrected/uncorrected values. I will double check to verify, but the pressure I got was probably the corrceted pressure. My question is now this... The station pressure was at about 3400'asl, while I was shooting at 3680'asl. Is there a way to correct for the elevation difference, or am I better off to just use the box "standard conditions at altitude" box?

Does it really just complicate things to try an deal with this pressure issure? and Is it "close enough" to just use the standard conditions box?

I am also getting the idea that I might be better off to just wait till I can shoot at 800 yards or so, and plug in the BC that gives me the correct marks. I just though that this would be a quicker way to get the same result. Wat do the majority of you guys think? Shooting or Chronographing?

Thanks for your help!!

Joel
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