BC issues

BB28MX · #1 06-13-2012, 03:27 PM

After shooting a chrono today and getting an MV of 2676 with 168 A-Max out of a 308 20 inch barrel with ES of 8 I am having troubles matching my drops with this info. My 680 yrd hold (not dialing and the scope at 12 power as it should to get accurate MILs) is roughly 3.8 MILs and my 1120 is almost exactally 8. In order to get this data from a program I have to use a g7 bc of .975! I'm at 3190 elevation, temp was 24.9 C, pressure was 29.67, and humidity was 18.7. Any ideas where I am going wrong?

Jordan Smith · #2 06-13-2012, 04:18 PM

I'm guessing that your reticle spacing was not in true mils. Assuming a 200 yard zero, your 680 yard dope should be about a full mil more than what you reported, and the 1120 yard drops should be about 4 mil more! That leads me to believe that you need to zoom the scope in a bit in order to get true mils.

A non-zero shooting angle at 1120 yards, etc, if not accounted for, may induce a little bit of error, as well.

rscott5028 · #3 06-13-2012, 04:52 PM

You indicated 12x to get "true MILs". Is that Mfg specs, or did you measure/calibrate it yourself?

What is your zero distance and scope height?

Did you zero the rifle and shoot your long range from the same position?

How many did you fire at each distance?

How did they group for each distance?

How did the measured velocity of each long range shot compare to the 2676 used for computing hold over?

-- richard

BB28MX · #4 06-13-2012, 09:43 PM

Ok so I figured it out... Target was at 605 not 685, and a more realistic g7 around .310 gives me correct holds for that distance. Guess I had too many numbers in my head but thanks for the help!

Bullet bumper · #5 06-13-2012, 11:09 PM

A JBM conversion of the G1 BC of .475 ( 168 A Max From Hornady site) . 168 grain , 2676 fps , to a G7 Bc = .239 G7

BB28MX · #6 06-13-2012, 11:19 PM

I just used that g7 bc and got 4.33 MILs for a hold but I was holding 3.8 and hitting fist sized rocks at what I have now confirmed to be 600. I could try to retest the power at which MILs are accurate. Is there anything else I should retest?

Bullet bumper · #7 06-14-2012, 03:17 AM

One single BC at one velocity can not model all that well compared to several BC's and velocities and also the trajectory is affected by local atmospheric and elevation conditions and angle of fire and cant .
The difference of .5 mill between real world drops and a ballistic program could be a result of some of the above changing .
Enter the actual measured angle of the barrel above horizontal the elevation above sea level , the temp & humidity at the time of shooting and see if that is a closer match to what drop you are seeing on the targets.
1 mill = 3.6 inch at 100 yards . So you could mark a target with that inch measurement and place it at exactly 100 yards from the scope sight . Then see at what power it corresponds to 1 mil on the reticule . If the scope is a second focal plane that is. If it is a FFP then it should be correct at all powers as the reticule grows with the power increase .
.5 mil is about 10.8 inch at 600 yards so make sure you are able to see that difference and aim with precision at 600 when shooting at rocks . Maybe a paper target may give more precision as to exactly what drop you are seeing as you can measure from the center of the group .
Fist size rocks at 600 yards would be around .5 MOA accuarcy that is very good.