Long Range Hunting Online Magazine


Go Back   Long Range Hunting Online Magazine > Rifles, Reloading, Optics, Equipment > Rifles, Bullets, Barrels and Ballistics


Reply

BC issues

 
LinkBack Thread Tools Display Modes
  #1  
Old 06-13-2012, 03:27 PM
Bronze Member
 
Join Date: Jul 2010
Posts: 33
BC issues

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?
Reply With Quote

  #2  
Old 06-13-2012, 04:18 PM
Silver Member
 
Join Date: Mar 2012
Location: Calgary, AB
Posts: 186
Re: BC issues

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.
Reply With Quote
  #3  
Old 06-13-2012, 04:52 PM
Platinum Member
 
Join Date: Apr 2010
Location: Allen, TX
Posts: 2,608
Re: BC issues

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
Reply With Quote
  #4  
Old 06-13-2012, 09:43 PM
Bronze Member
 
Join Date: Jul 2010
Posts: 33
Re: BC issues

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!
Reply With Quote
  #5  
Old 06-13-2012, 11:09 PM
Gold Member
 
Join Date: May 2009
Location: Australia
Posts: 736
Re: BC issues

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
Reply With Quote
  #6  
Old 06-13-2012, 11:19 PM
Bronze Member
 
Join Date: Jul 2010
Posts: 33
Re: BC issues

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?
Reply With Quote
  #7  
Old 06-14-2012, 03:17 AM
Gold Member
 
Join Date: May 2009
Location: Australia
Posts: 736
Re: BC issues

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.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes


Similar Threads for: BC issues
Thread Thread Starter Forum Replies Last Post
TAC 15 Issues jbmm411 PSE TAC 15/15i Crossbow Hunting Forum 7 12-08-2010 03:13 PM
Accuracy Issues Huvik Long Range Hunting & Shooting 5 09-14-2010 05:48 PM
having issues foreign Bowhunting 1 01-06-2010 10:32 PM
22-250 issues- help? peppy1hunting Reloading 2 11-28-2008 09:35 AM
Scope Issues!!! ktmhk53 Long Range Scopes and Other Optics 3 03-22-2003 07:52 PM


All times are GMT -5. The time now is 01:15 PM.


Powered by vBulletin ©2000 - 2014, Jelsoft Enterprises Ltd.
Content Management Powered by vBadvanced CMPS
All content ©2010-2014 Long Range Hunting, LLC