Too many MOA in scope base?

May 7, 2005
261
7
18
WA
#1
Is there such a thing as having too many MOA in a scope base or any issues associated with it? I recently replaced my 20MOA base with a 40MOA base and rezeroed. Now looking through the Kahles it seems kinda off and I can't seem to figure it out. I don't have any experience with anything beyond 20MOA bases.
 

flyer

Sergeant of the Hide
Apr 25, 2018
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#2
I have a scope with about 50 MOA total elevation adjustment so I had to choose a 20 MOA base instead of 30 MOA because I wouldn't have been able to zero otherwise. Now I have around 45 MOA of usable elevation travel below my zero instead of 25 MOA if I used a base without slope.

If you can still zero, you should be fine.
 

CANTHITSH!T

New Hide Member
May 12, 2018
35
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#3
Is there such a thing as having too many MOA in a scope base or any issues associated with it? I recently replaced my 20MOA base with a 40MOA base and rezeroed. Now looking through the Kahles it seems kinda off and I can't seem to figure it out. I don't have any experience with anything beyond 20MOA bases.
Since looking through an optic at any kind of a major angle induces parallax and the light passing through an optic can't really enter your eyeball straight if the scope is angled significantly and you can't/won't align your eyeball with the ocular lens, yes its possible to have "too much MOA" under an optic. Elevation adjustments within the scope change the "apparent location" of the image while your eye stays put. MOA bases actually angle the optic relative to your eye and you have to get your eyeball back in line with the light path to make things look "right".

If you look at the products made/sold by longtime gun/optics manufacturers you'll typically find they max out MOA at 15-25 MOA and the mount/rail is going to "use up" a fair amount of that just in zeroing the optic at the relatively short ranges shooters of the tactical variety seem to be determined to zero at and the angle will come close to bottoming out most optics when zeroed at those ranges or at least will move the elevation "down" to around its maximum before windage adjustment really begins to be limited.

Its also worth mentioning that the "MOA" of a rail/mount isn't "fixed" and is relative to the height of the scope centerline or "sight height" above bore centerline so the rifle, caliber (bore diameter), other mount(s) and the size of the scope tube affect how much actual "MOA" a given "angle" will produce. The "15 MOA" rail on my AR-50A1 has noticeably more "angle" as does any scope mounted on it using "level" mounts than does the Nikon 223 XLR "20 MOA" one-piece mount I've had on an AR15 and now have on my LR308. The "sight height" with the latter is WAY lower and therefore far less "angle" is needed to create "20 MOA" of "elevation change" with it than with the super-tall "15 MOA" rail on my .50.

Barrel length, "zero distance" etc also change how much "MOA" a given angle produces/gives and the higher the mount the more "MOA" you'll use "zeroing" at short-range like 100 yards. That may seem like a good thing but since you're always giving up SOME "MOA" using a scope because its centerline is well above the bore centerline, a "high MOA" mount/rail can easily put your elevation adjustment so far "down" at 100 yards you're into "windage bind" territory and may have to dial in more "windage" just to force the scope to "zero" at that point so when you "dial up" for longer distance, your windage "moves" all by itself.

The other situation that's possible with a high rail/angle and short "zero distance" is that error in the alignment of the scope "vertical centerline" with the bore vertical centerline is minimized and less adjustment of windage is needed to compensate for it at "close range" leading to major "error" at longer ranges since ANY error in that alignment is "too much" and windage error affects elevation error for the same reason "cant" does. A scope should shoot "dead enter" horizontally when properly bore-sighted and if you have to make windage adjustments at close range just to get "zeroed" there's an issue with at least one part of your "system". Eye relief on scopes exists to ensure that the shooter is able to "center" his or her eyeball behind and with the scope centerline to maximize "sight radius".

The first step in mounting any scope and one hardly ever mentioned in "how-to" videos/posts/articles is to center the adjustments of both elevation an windage by "counting clicks" from "maxxed out" up/down and left/right to the other extreme, dividing by two and moving the adjustments the resulting number of "clicks" from one extreme or the other. A good "test" is to do an "old-fashioned" boresighting and "center" the adjustments from BOTH extremes and compare where the reticle ends up doing both.

If the scope is "centered" and the target being actually "boresighted" looking through the bore isn't centered in the scope and/or the reticle isn't horizontally centered on the target with the target centered in the bore, you've got problems. And since a good old-fashioned boresighting is done at 25-30 yards or so, just think how far "off" any deviation from center is at multiples of that distance. For "long-range shooting" I'd be more interested in and put more importance on a "wide target test" than a "tall target test".
 
Last edited:

Clearlight

Full Member
Jul 23, 2014
649
175
43
Brisbane Australia
#4
If you managed to get a 100 zero , you are looking through an optic at the edge of its operational
range . Dial up 40 MOA , look at the same target and most likely it will appear clearer .

I’m happy to sacrifice clarity at 100 , for an optic that will dial further and be in the optical ‘ sweet
spot further out . The only way to keep from bending the light path to the point of distortion ,
is the have an external elevation adjustment of some type . We will see more of this in the
near future .
 
Likes: MontanaMarine
May 7, 2005
261
7
18
WA
#5
When I initially zeroed with the Kahles I lost 9 mils of elevation to the zero topping out at 16.5 mils. Moving to the 40MOA rail got me 6.4 mils back and now I top out at 22.9 mils which is substantial.
 

mijp5

Gunny Sergeant
May 7, 2009
3,931
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#6
I originally had a 45 MOA base for my hensoldt 4-16 which mathematically should have worked, assuming 26 mils of total elevation. Turns out, the scope actually has about 24 mils, so I was exactly 1 mil high on a 100 yard zero.
 

Fig

Janitor of the Hide
Mar 15, 2018
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The Most Dangerous City in the USA
#7
When I initially zeroed with the Kahles I lost 9 mils of elevation to the zero topping out at 16.5 mils. Moving to the 40MOA rail got me 6.4 mils back and now I top out at 22.9 mils which is substantial.
What cartridge are you shooting?
I shoot a 6X47L, and my Khales revs out @ 16.9 with a 20 MOA base which is just on the second rev. The cartridge goes transonic @ 1,375, which is 13.6 mils. I figured I'd only be shooting to 1,400 yds, so this was good.

Last weekend there was a 36" target @ 1,500 yd stage. I was @ 16.5 mils of elevation and 5.7 for 15-25 mph wind (also almost reved out). I had BARELY enough to dial it.

I was on. My wind guess was correct. I was hitting above and below it from the first shot, but no joy. I would correct slightly, and then miss the other way. I zeroed that stage and was disappointed, but a lot of people also zeroed it. I would have liked to get an impact just to say I did.

I have thought about a 40 MOA rail, but with my current rig it just won't buy me much, and I'm afraid of sacrificing precision closer as CANTHITSH!T points out. I have read that no scopes like to be zeroed too close to where they bottom out.
 

regency

Sergeant
Nov 16, 2007
295
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Memphis, Tennessee
www.patwar.com
#9
Guys, would I be safe with an AI rifle that has 20MOA built in to the rail, and then my once piece scope mount has another 20moa in it as well?

It's .338 and will mostly be shot in the 600-1800yd range... but would like to have a 100yd zero with it.
 

mijp5

Gunny Sergeant
May 7, 2009
3,931
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#10
Guys, would I be safe with an AI rifle that has 20MOA built in to the rail, and then my once piece scope mount has another 20moa in it as well?

It's .338 and will mostly be shot in the 600-1800yd range... but would like to have a 100yd zero with it.
How much elevation is built into the scope elevation turret?
 

dinc

Sergeant
Feb 25, 2007
331
0
16
Oregon
#13
People also tend to forget that a scope tube is round. When you adjust windage and elevation you're thinking box. But as you get toward the top or bottom of you elevation adjustment, you also lose some of your side to side windage adjustment. People ran into this along time ago when scoping AR's. As the barrel has extension screwed onto it and then this is all put into the upper receiver there can be alignment issues with the upper. The Ruger Precision Rifle has this in spades. Sometimes you have to use a lot of windage to zero. Then when you try make big elevation changes you can prematurely hit the side of the tube and as it moves vertically, it is also forced toward the center of the tube. And the movements become unreliable.

Something to think about when making extreme elevation adjustments, both when mounting and dialing.

But as mentioned the more frequent effect of high MOA mounts is moving in and out of the optical optimal center of the scope.

Lastly, if a scope spends a lot of it's time setting with the elevation knob cranked all the way down, like zeroed at 100 yd with a 40 moa mount, the erector spring is going to spend a lot of it's time compressed. There is a possibility of premature erector spring failure, increasing inversely with scope quality.

Shoot a lot and you'll overcome all these things. Go shoot.
 

SageRatSafaris

New Hide Member
Jun 13, 2018
76
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Oregon
sageratsafaris.com
#14
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