JBM ballistics calculator question

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by CRNA, Feb 16, 2011.

1. CRNAWell-Known Member

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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. KRPWell-Known Member

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Doesn't matter, cant angle in JBM is something else.

3. CRNAWell-Known Member

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Ok, thanks for the reply. I figured if it mattered they would have had an input for it.

4. nheningeWell-Known Member

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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. CRNAWell-Known Member

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I have 1.125" rings. That will be the number that I input right?

6. nheningeWell-Known Member

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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. TnTomWell-Known Member

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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.

8. nheningeWell-Known Member

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better defined than I did...

9. CRNAWell-Known Member

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Thank you for the info guys. I really appreciate it!!!!