Range Report Calculating BC with numbers from Labradar

[Kyskeet]

I am shooting bullets with no verified BC. 136gr EPS BTHP in 6.5 Creedmoor. They are very consistent weight and base to ojive length. I used the muzzle velocity 2805 and 100yd velocity 2640 f ok my Labradar to calculate a G7 on JBM sight and got .271. I'll be curious to see how this works out when plugged into a ballistic calculator. Question: anyone arrive at a unverified BC similarly?

 

[_Raining]

I am shooting 140 gr Berger Hybrids which according to Applied Ballistics App that has a G7 BC of .32

My Labradar shows a muzzle velocity of 2857 ft/s and 100ft velocity of 2811 ft/s (Silly me had it set to feet instead of yards so I only have values from 0 to 100 ft so for applied ballistics app I used 33 yrds, which is close enough to 33.3333)

Applied Ballistics shows 33 yrd velocity of 2810, right on the money.

So what does JBM say?
Near Velocity: 2857 ft/s
Far Velocity: 2811 ft/s
Chrono Separation: 100 ft
Drag Function: G7
Temperature: 85 F
Pressure (Non Corrected): 30.1
Humidity: 63%

This results in .330

Using the rest of the Labradar values:

Set Far Velocity to 2823 and that separation to 75 ft. We get .336

Set Far Velocity to 2835 and that separation to 50 ft. We get .346

Set Far Velocity to 2844 and that separation to 30 ft. We get .352

Set Far Velocity to 2851 and that separation to 15 ft. We get .382

Now if we set Far Velocity to 2715 and separation to 100 yrds (This is what applied ballistics has for G7 BC of .32) we get .319

So it looks like JBM has a trend of starting out with a high G7 BC when using close values from Labradar and gets very close to what Applied Ballistics has at 100 yrds (Theoretically, as I don't have Labradar values for 100 yrds).

So I would say you are safe to use the JBM G7 BC when using a Labradar value of 100 yards. This weekend I will be redoing some load development so I can update you with what the Labradar shows at 100 yards. You could also do the same as me and see what JBM says for all the values shown by the Labradar, and see if it shows the same trend that I see which is the BC starts high and shrinks down the further you go out.

 

[Kyskeet]

Thanks for the info, will be interested in your work up infor

 

[ptosis]

Though about it; wouldn't work -- the measurement distance is insufficient.

 

[Forward543]

Your bc will change as the velocity changes. The farther out it gets the more it changes. Get your initial, then shoot as far out as you can. Adjust your bc according to your drop.

 

[_Raining]

My labradar wasn't picking up velocities past 50 yrds so unfortunately I have no additional data for you but as Forward543 said, use what you got and adjust it as you shoot further out.

 

[damoncali]

I don't have a labradar, but I've been wondering if you could use one to quantify lot-to-lot variances in BC. The distance measured is far too short to make accurate BC calculations, but it seems like it *might* be possible to make relative comparisons.