Range Report MV, chrono, and Applied Ballistics huge discrepancy...

[Ryguy]

I have an RCBS chrono telling me my MV is 2822 fps at DA 5300 ft. With a DA of 5500, the applied ballistics app trajectory calculation was off by 3.00 MOA at 904 yards. The calculation from applied ballistics called for 22.25 when the actual call was 19.25. Shot angle, DA, wind, temp, humidity, Coriolis, and everything was accounted for. I worked up the muzzle velocity until it matched 19.25 at 904 yards and it called for 2935 fps. The applied ballistics app was dead on at every distance after that. Why the discrepancy and which MV is most likely correct? Are chronometers frequently inaccurate by 100fps? Thanks.

 

[Ryguy]

Well, BC and drag model certainly could be the problem. However, I was using Brian Litz's drag model and it was interesting how a change in MV aligned every distance with the correct elevation correction from 100-904. Also, another consideration was the wind correction. My Kestrel said wind was blowing 4 mph at 270 degrees which applied ballistics calculated a 2.1 MOA correction. The actual correction came out to 3.25. Granted the wind could have changed drastically down range but the mirage was telling me it was about 4 or so the entire way. edit: never mind about the wind, a 6mph wind calls for 3.4 MOA and I am certain that I am not capable of discerning a 4 from 6 mph wind via mirage.

 

[steve123]

Yes, chrono' s can and do lie, likely the 2935 fps is correct.

 

[Fester47]

Yep. They can and do. But not 100 fps, no.
A chronograph that has a 100 fps error is scrap metal.

I agree my Crony reads 50+ fps slower than what my dope shows it should be.

 

[Ryguy]

Another question is how accurate are ballistics calculators, this one in particular? Is the RCBS Ammomaster chrono known to be junk?

 

[Lowlight]

Ballistics Calculators are very accurate, more accurate than the shooter in most cases, but garbage in equals garbage out and they are not Clairvoyant.

I would recommend screenshots of what you used, but chances are you played with it far too much before using it and knocked something out of whack. It's probably a combination of errors, with all the crap turned on you are compounding errors with unnecessary data that might not be inputted correctly.

Finally, do not discount sizable errors in your chronograph, they are not all created equal, and is certainly a bigger variable than the ballistic solver itself.

 

[Ryguy]

"Garbage in equals garbage out.." Man, I haven't heard that since undergraduate statistics. I understand fully what each of the entry fields are in the app and I am confident they were entered correctly. Thanks everyone, I am going to go with my chronometer was off since a single variable correction brought everything in perfect alignment.

 

[steve123]

Yep. They can and do. But not 100 fps, no.
A chronograph that has a 100 fps error is scrap metal.

Ha, funny you should mention that - scrap metal, remove the s and you have a more correct alias. Hmm, which folding chrony resembles this description? Had two and a friend had one. None of them gave correct FPS or consistent FPS and it wasn't till we bought a Oehler that we found this out. All we knew is that we couldn't get our dope to line up. Yes the worst was 100 fps off and more at times.

 

[mil_coder]

Ballistics Calculators are very accurate, more accurate than the shooter in most cases, but garbage in equals garbage out and they are not Clairvoyant.

I would recommend screenshots of what you used, but chances are you played with it far too much before using it and knocked something out of whack. It's probably a combination of errors, with all the crap turned on you are compounding errors with unnecessary data that might not be inputted correctly.

Finally, do not discount sizable errors in your chronograph, they are not all created equal, and is certainly a bigger variable than the ballistic solver itself.

Totally agree w/ Frank here. If you post what your inputs are, I'd be happy to walk you through what your inputs look like. More than likely, something is a little off. One thing to keep in mind is that on the Kestrels (if you are using one), then it has aerodynamic jump calculations built into it. So if you have the wind direction completely wrong as if it's blowing from left to right and it's actually blowing right to left and you have a value entered that is too high for the wind, then I could see where it could be up to 0.5 MOA off.

 

[AtOne]

Build another bullet profile using the opposite G version your already using.
If your using G7 make one with G1 and see if it lines up better from your chrono and actual data.

 

[Ryguy]

The data I was using are attached. The only thing missing is the shot angle of 5 degrees and Coriolis which I was using, 36.14 (Nellis AFB) with an azimuth of 59 degrees. Again, everything lined up perfectly from 100 to 904 leading me to believe the corrected variable of MV was the culprit because, aligning perfectly like that is way outside of chance if it was a different variable. (T-Test Independent Sample).