20 MOA sight base?

[beeker77]

Finally broke down and ordered a nice scope (Sightron SIII 8-32x56), awaiting delivery.
Several buddies said that's a good scope, but to shoot in 1,000 yard events, it will require a 20 MOA base set, because the scope has only 70 MOA of elevation travel, and 90 MOA is needed to go out to 1k yards.
I installed the 20 MOA base (no scope yet), ...so how does it work? To me, it just sets the scope about 1/2 inch higher up off the receiver. How does this gain 20 MOA? Can someone explain in simple terms, please?

 

[Sharkbite]

Good question...

The back end of the base is 20moa higher then the front end. This gives the scope a "down" angle or the bore an "up" angle

This allows yor to zero the scope lower in its adjustment range and therefore have more room to "come up" for distance shooting

 

[beeker77]

20 MOA question

Thanks for the response, Sharkbite.
I totally understand that explanation, and that is exactly what I expected should be the result of the installation.
However, when I check the level of the sight bases top surfaces, with a straight-edge resting across both bases, the straight edge is perfectly level, and perfectly parallel with the barrel axis. So unless the angle is really, really subtle, I just don't see that there is any change potential in the scope axis; it's still the same as if it were mounted on 'normal' sight bases.


BTW: The product is Nightforce "Winchester Long Action" 20MOA Scope Bases, two-piece, Item ID NFW70LA.

 

[beeker77]

Tapered 20 MOA sight base

Mystery solved. It IS a very subtle taper!

Using digital calipers, on the front base, the rear edge rises 12.37mm from the receiver, while the front edge rises only 12.22mm, for a downward taper of .15mm, or about 01.2%. Assuming the rear base shows the same percent of taper (I have not checked), apparently that is a sufficiently downward angle introduced into the scope installation to produce a 20 MOA gain.
Does this make sense?

 

[Bart B.]

1 MOA is equal to 1/3600th of a distance. A point on the top of a 20 MOA base 6 inches back from another will be 6 / 3600 X 20 = .0333 inch higher.

Percentage wise, .0333 / 6 = .00555; 0.555%.

A 1.2% taper is about 41 MOA.

 

[Sierra280]

Also, keep in mind that different scopes have different amounts of internal adjustment. Sight bases do come in a variety of elevations, I think badger ordinance has upto 30 or 35 MOA. I got a leupold 15 MOA base because that's probably all I'll ever need (plus I'm kind of a brand snob). It's better to have it and not need it though.

 

[Bart B.]

Good point.

Base angles shouldn't be more than 1/3rd the scopes adjustment range. That allows for receivers scope base mounting axis being several MOA off from the bore axis so you can still get a 100 yard zero. That happens more often than most folks realize. If they're well aligned, a greater angled base could be used.

 

[Jim Watson]

But why do I need a 100 yard zero on a 1000 yard rifle?
If I can get on at 600 and 1000, I don't care if it shoots high at the 100 or 200 yard club range for testing loads.

 

[Bart B.]

Some folks like to practice shooting prone at their local 100 yard range instead of driving 200 miles to one that goes past 600. Then see how close or far the bullet goes from where they call the shot. Excellent way to practice without annoying cross winds to deal with. It's best to master basic marksmanship skills before mastering wind reading and compensation skills.

Others want to get a "shooting bore sight" by first getting a zero at 100 yards then adjusting the sight down to change group center down an amount on target equal to bullet drop plus sight height above bore axis. Then zero the sight knobs and adjustment scales. After they've calculated bullet drop for another load, it's easy to set the sight to the right value for each range it'll be used at when the elevation knob/scale starts at zero. I've done this and it makes things easy.