Rifle Scopes 20 MOA Base Calculation Confusion

[mrgreentie]

Let's say I have a 20 MOA base and my optic only has a 50 MOA of elevation (25 up and 25 down). And let's say my ballistic calculator says I have 32 MOA drop at 1,000 yards...what do I do? Will I have what I need to get there? Should I account for the 20 MOA in my initial zero?

When I add the base, do I click my elevation as far down as it goes then zero my rifle? Basically adding 20 MOA to the optic max of 25 MOA up giving me elevation max increase of 45 MOA? OK...I'm confusing myself.

What I'm asking is this, how is the 20 MOA base added to my elevation potential and how is it compensated for when it's time to take the long shot?

Thanks for any help you can provide clearing-up this confusion!

~Mac

 

[Lindy]

Re: 20 MOA Base Calculation Confusion

OK - let's start with the basics:

There are two angular components to a short-range - say, 100 yard - zero.

One is compensation for the height of the scope over the bore. That angle may be computed as:

angle = arctan (height above bore / distance to target)

In the case of my AI, for example, with a height above bore of 1.75 inches, at 100 yards (3600 inches) it's:

angle = arctan(1.75 / 3600) = .03 degrees or 1.67 MOA

The other component is the drop from the muzzle to the zero range. With my load at 100 yards, that's about 2.7 inches, or, using the same formula above, about 2.58 MOA.

Add those two up, and it's about 4.25 MOA or about 1.25 mil.

Now - let's say your scope has 25 up and 25 down, and it's sitting on a 20 MOA base.

With no zero, and the reticle centered, you'll have about 45 up - 25 + 20 - and about 5 down.

Once you rezero, you'll lose about 4 up, as noted above.

That means you'll have about 41 up, and 9 down.

It may not be exactly that, because scopes often don't have exactly the manufacturer's specification. But it shouldn't be far off.

 

[Goin'Hot]

Re: 20 MOA Base Calculation Confusion

Put on the 20 moa base. Assuming the reticle in your scope is perfectly centered, rezero your scope @ 100 yds. You now have 45 minutes of up elevation avalible. If your shot requires 32 moa, dial in 32 moa on your scope and take the shot.

Don't over complicate the issue.

 

[Austan]

Re: 20 MOA Base Calculation Confusion

Adding a 20moa base will give you 20moa more up adjustment. In a perfect world you would have to (from mechanical zero (25moa up, 25moa down)) move the reticle 20moa down to zero at 100, thus giving you 45 moa in up elevation available. You will then have your 32 up to get to 1k plus extra to go farther out given the opportunity.