# Help calculating a BC from two speeds

Discussion in 'Long Range Hunting & Shooting' started by aroshtr, Oct 2, 2007.

1. ### aroshtrWell-Known Member

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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.

2. ### royinidahoWriters Guild

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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.

Last edited: Oct 2, 2007

3. ### Jon AWell-Known Member

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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.

4. ### milanukWell-Known Member

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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&quot; Hg this morning), or raw ('station') pressure (28.58&quot; Hg). The NOAA data for Raton (6350' ASL), as an example, is only available as corrected pressure (30.13&quot; Hg). The actual uncorrected pressure at the range there (6600' ASL) is more like 23.7&quot; 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

5. ### Jon AWell-Known Member

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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.

6. ### royinidahoWriters Guild

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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.

7. ### aroshtrWell-Known Member

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Oct 2, 2007
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?

Joel

8. ### milanukWell-Known Member

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Again, I don't normally calculate B.C. from velocities (too chicken about hitting my sky screens!), but generally speaking, a couple hundred foot difference in altitude makes *very* little difference for calculating come-ups, etc. I would imagine the same thing applies here.

Splitting the difference (3500' ASL) and using the data you provided, plugging it into JBM (link provided above)... nets a BC of .519, very close to what Jon came up with. I'd guess somewhere between 0.515 and .520 should get you going.

Monte

9. ### aroshtrWell-Known Member

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Thanks a Ton! I will use the .515-.520 for testing, and see if there is a need to make a change.

Have any of you guys had any experiance with Kenton Industries for ordering custom knobs? Good,Bad,Other options?

Joel

10. ### milanukWell-Known Member

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Again... no direct experience.

What happens if you change locations for a hunt or a match, and end up some place where the environmental conditions are very much off from what you had the knobs made for? For me, a Kestral and a PDA w/ Exbal (or something similar) works pretty well, or at a minimum a printed out drop chart on the side of the stock.

YMMV,

Monte

11. ### jwp475Well-Known Member

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+1...........

12. ### royinidahoWriters Guild

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Well folks, I didn't do for squat on that little exercise.

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I have a set of Kentons that I used with good success. IF you are going to be using them for one set of conditions and you know you will continue using the same load they are real handy.
Once you have done all of your "field verified drops" homework ahead of time.
Now I lean toward using a reticle that is listed in MOA that will work in any condition, with any load, and on any gun.
And I also use the turrets also.

14. ### Tom in SWNYMember

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I have to agree with Roy. A boattail with all those numbers taken into consideration comes out to .340 as a g5 bc.