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# JBM ballistics calculator question

#1
02-16-2011, 06:58 PM
 Gold Member Join Date: Jan 2010 Location: NC mountains Posts: 504
JBM ballistics calculator question

I have a 20MOA base on my rig. Does one enter this information in JBM? I see an input for cant angle, but it's in degrees.
#2
02-16-2011, 07:10 PM
 Gold Member Join Date: Dec 2007 Location: Kuna, ID Posts: 594
Re: JBM ballistics calculator question

Doesn't matter, cant angle in JBM is something else.
#3
02-16-2011, 07:16 PM
 Gold Member Join Date: Jan 2010 Location: NC mountains Posts: 504
Re: JBM ballistics calculator question

Quote:
 Originally Posted by KRP Doesn't matter, cant angle in JBM is something else.
Ok, thanks for the reply. I figured if it mattered they would have had an input for it.
#4
02-16-2011, 07:47 PM
 Silver Member Join Date: Oct 2008 Location: Bryan, Tx Posts: 284
Re: JBM ballistics calculator question

No it doesn't matter for a 20MOA rail. What does matter is your zero point. This compensates for the 20 MOA rail. Don't forget an accurate scope height too!
#5
02-16-2011, 08:05 PM
 Gold Member Join Date: Jan 2010 Location: NC mountains Posts: 504
Re: JBM ballistics calculator question

Quote:
 Originally Posted by nheninge No it doesn't matter for a 20MOA rail. What does matter is your zero point. This compensates for the 20 MOA rail. Don't forget an accurate scope height too!
I have 1.125" rings. That will be the number that I input right?
#6
02-16-2011, 08:20 PM
 Silver Member Join Date: Oct 2008 Location: Bryan, Tx Posts: 284
Re: JBM ballistics calculator question

No. Roughly put, you need the distance from the center of your scope to the center of your chamber. Several formulas are available in different threads. You can estimate with a micrometer, or calculate the actual measurements and plug in to formula (i.e. 1/2 scope diameter plus 1/2 chamber diameter plus distance between bottom of scope and top of chamber). This is the actual starting point of the bullet so this height is pretty important.
#7
02-16-2011, 08:29 PM
 Bronze Member Join Date: Dec 2009 Location: middle tn Posts: 72
Re: JBM ballistics calculator question

Measure the dia of the barrel below the bell, divide by 2.
Measure the dia of the bell, divide by 2.
Measure the distance from the top of the bell to the top of the barrel and subtract half the dia of the bell from that dim and add half the the dia of the barrel.

That will give you the distance from the center line of the scope to the center line of the bore. Enter that number in your JBM calculator.
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