JBM Ballistic Calculator question

[squirrelsnpr]

I was wondering where I input the 20 MOA rail I have. I see a "line of sight angle" heading and wondered if this is it? If so, what angle is 20 MOA? And would it be considered shooting uphill for a positive number or downhill for a negative number.
Thanks,
Mitch

 

[GermanS1]

Mitch, 20 moa is an angular measurement, ther are 60 minutes of angle in a degree. However, I think the program is asking about the angle at which you are shooting (uphill-downhill) not about the sight base, so you don't really input that 20 moa anywhere. Us positive for uphill, negative for downhill.

 

[squirrelsnpr]

but wont the output numbers be off b/c my scope is angled?

 

[queen_stick]

Trust me, I understand how confusing it can be.

The drop chart produced by JBM, or any other ballistics calculator, is based on bullet trajectory, not scope angle. The reason there's no place to include the angle of the sight is because it's using your 'zero' range. If you zero your scope so the POI and POA are the same at 100 yards (zeroed at 100yds), then it doesn't matter what angle your sights are at - you're still zeroed at 100yards. From that point, adjusting your sights to correct for elevation is independent of the angle of your sight (scope). Basically, if you zero a scope/rifle for 100yards and need to adjust the sight for the drop at 300 yards, you click up 3 MOA (as an example). If you're using a 20 MOA base, you still need to use the exact same 3 MOA elevation value to adjust for the 300 yard zero.

20 MOA bases give you more elevation adjustment in your scope, it doesn't change the "MOA-per-click" on your scope.

hopefully that makes sense.

 

[~Ace~]

To make it Simple.... JBM is a External Ballistics Calc. With the exception of the Scope height above bore used as a base setting, it only considers what happens AFTER the projectile exits the barrel.

Your 20 MOA base has no effect on the ballistics, trajectory or anything else besides adding usable elevation to your optics.

As a side note, because of the slight angle the mount has, when measuring Scope height above bore, measure in the middle of the scope where the erector is for the most accurate figure... not at either end.