Author Topic: BC and other things (Read 712 times)

Alan · « **on:** July 09, 2017, 05:15:33 AM »

My .25 Cobra whistles 25.4 grain AirArms at 876 ±2 max. It does so almost monotonously. Accuracy is better than anyone should expect. If the wind isn't blowing, 75 yard groups can be covered with a nickel. I couldn't be happier. However, it has me wondering about the listed BC for the pellets.

The BC for JBS 25.39 grain pellets is listed as .036. The AirArm version should be the same. I haven't measured the down range velocities (that will happen soon), but considering the drop of the pellets I'm seeing, the BC is very close to .040. I came up with this by playing with the BC "box" in Chairgun. Setting the BC at .040 seems to duplicate the drops I'm seeing.

If I could get the Labradar to work correctly, I might be able to come closer. Nonetheless, if Chairgun it correct, then the BC of .040 is closer to reality.

Any comments?

rsterne · « **Reply #1 on:** July 09, 2017, 08:28:40 AM »

It is extremely hard, from my experience, to get two accurate POIs, which is what you need to determine the BC from them.... Even a tiny error in measuring the center of the group will give a fairly large error in the BC.... In addition, you need to correct for your atmospheric conditions.... The biggest change will be altitude, if you are not at sea level.... Make sure you use the Environmental Applet to input your altitude, barometric pressure, temperature and humidity.... We are at 2500 ft., and if I measure the BC as 0.05, it ends up at 0.04 corrected to sea level, as it should be....

This is not to say that your BC is not 0.04, that is within the range of values I would expect for that pellet.... just that you need to be careful to de everything correctly, it's easy to forget to correct for altitude, for example....

Bob

Alan · « **Reply #2 on:** July 09, 2017, 09:23:37 AM »

That could account for it, as we're ≈3,500 here in Roswell. I have measurements at 25, 50, 75, and 100 yards, but what I really need is the velocity at those distances.

I used a Labradar belonging to one of the gun club members, but neither of us could get it to work as advertised on pellets. A new microphone is on its way, which I suspect was some of the problems. Entering the altitude and pressure, puts the BC at nearly .060 if I did it right. Next week will tell as the new microphone should be here.

rsterne · « **Reply #3 on:** July 09, 2017, 11:47:41 AM »

That doesn't sound right.... There is less air density, and hence resistance, at 3500 ft.... so your calculated BC will be too high.... When corrected to sea level the BC will drop, not increase....

If you are inputting the barometric pressure corrected to sea level, you must click on the button for "Mean Sea Level Pressure".... If you are inputting the ambient barometric pressure, then click on that one.... For example, we are at 750 m here (2461 ft).... The last time I shot, the corrected MSLP was 1014 mBar (29.95").... If I used ambient pressure I would have had to enter 911 mBar (26.91") and clicked the other button....

Inputting the data correctly, for the last BC I did, I get a corrected BC of 0.036.... If I forget to use my local altitude (which is what makes the biggest difference on a "normal" day), but leaving the other things the same (pressure, temperature, humidity), I get an (incorrect) answer of 0.040....

Bob

Alan · « **Reply #4 on:** July 09, 2017, 02:43:57 PM »

I used the local pressure, temperature, and humidity. When I enter all of that, in order to get the drop on Chairgun equal to what I'm seeing on the range, I have to set the BC somewhat higher, ≈.06. If I use corrected pressure, but leaving the temperature and humidity the same, I get .040. I think this agrees with yours.