0 vs 20 MOA Mount

['88Scrat]

Building an M110 clone (ish) and need help with the scope mount and whether a 0 or 20 MOA base would better suit my needs.

Rifle = HK MR762A1
Optic = Nightforce ATACR 4-16x42 FFP Mil-XT

I don't ever really plan on taking it out past 600 yards but based on my internet sluething that seems to be kinda right in the grey area for where a 0 mount and 20 mount intersect.

Is there any downside to a 20 MOA mount? Or for my purposes would a 0 mount work just fine and I'm overthinking it?

Thoughts?

[Nathan]

Your optic has elevation adjustment for way past 1000, I believe. The 20moa is to get you more centered around 1000 for better optical performance and windage adjustment.

I’d rather have the 0 moa, until 1000 yds is a regular shot distance.

[tangolima]

Can't see much down side of it doesn't cost anything extra. 5.56 is going to clung out around 600yd. You probably will never need the extra 20moa, but the optics always have chance to go to other rifle for longer range applications.

-TL

Sent from my SM-N960U using Tapatalk

[Shadow9mm]

There is no downside to the 20moa mount imho, its all i buy anymore. It will work just fine with normal ranges and zeroes, but give you a lot more room to push out further later on, its future proof.


That scope has 89moa of adjustment. If your reticle is centered from the factory that means you can move up or down 44.5 moa from center. Generally most adjustment is up to compensate for bullet drop, so most of the 44.5 down adjustment will never be used. Adding in a 20moa mount means you now have 24.5moa of down, and 64.5moa of up adjustment. Thats not even accouningt for the 2-6moa up you will probably use while zeroing.

[stagpanther]

Basically a 7.62 x 51 SASS with milrad scope? Probably no biggie either way I would think. I like reticles that give me an additional range of elevation and windage that I can adjust on the fly rather than fiddle with turrets.

[bamaranger]

My limited experience is you may or may not need need the extra the 20MOA base offers to get to 1000yd with the .308. Depends on how the rifle zeros and what bullet weight you choose to shoot. The big wind slipping .30 cal slugs, 175 gr plus, need a good bit of elevation to get out that far and there's no telling how much elevation you'll need to get zeroed for what ever distance you set the the system up (zero at 100.. 300....600?)

I realize you state 600 is supposedly your max anticipated, but why sell the rifle short? A 20 MOA base will put your 600 yd dope somewhere in the middle of the scopes adjustment range, the optimum place to be. Plus you will have plenty of elevation to get to 1000 or further with about any bullet weight no matter how much elevation you might need for initial zero. I would not think the price would be much different.

['88Scrat]

Sounds like what I'm hearing is there is really no downside to a 20 MOA mount.

I just figured that like everything there would be a tradeoff of some sort.

Harder to compensate for a target at 100 or 200 yds for the tradeoff of being able to shoot to 1000 yds for example.

Just me overthinking it probably...

[Shadow9mm]

Just points the scope down a little. Zeros and shoots like normal. Won't effect anything after you zero up except you will be able to use more of your scopes up adjustment.

[Koda94]

Originally Posted by '88Scrat

Sounds like what I'm hearing is there is really no downside to a 20 MOA mount.

I just figured that like everything there would be a tradeoff of some sort.

Harder to compensate for a target at 100 or 200 yds for the tradeoff of being able to shoot to 1000 yds for example.

Just me overthinking it probably...

I dont know if this was a thing for older scopes or new but Ive heard its better for a scope to remain closer to its optically centered position for shooting. So basically if your -not- going to shoot past 1000yds then a 0moa rail is better since most shooting is done at 100yds.

[Unclenick]

It varies with the scope, but generally, the nearer you are to using the centerline of the optics, the better the image quality. The higher the quality of the optics, the less severe the loss of image off-center is, but at high magnification, in particular, you may see the loss of brightness or contrast. Also, generally, the further below center you have to lower the vertical, the more stress you put on the springs that the adjustment screws compress, so they can take a set and cost you adjustment stability at the opposite end of the range, but that takes a while.

Assuming you have the 16.5" barrel they show, you would be launching Federal GMM308 (168-grain bullet) at a muzzle velocity of about 2434 fps. If your scope centerline is 3¼" above the bore centerline (about what the scope in the owner's manual is), using +20 moa mount when the scope adjustments are at middle setpoint, your elevation will be zeroed at 505 yards. Other, relative to the optical center for that load, would be:

100 -13.9 moa
200 -12.1 moa
300 -8.9 moa
400 -4.9 moa
500 -0.2 moa
600 +5.3 moa

I think, based on that, I would be more inclined to buy a mount with 15 moa of elevation, so you stay closer to the optical center on average over your range. A 15-moa base will have you looking straight through the middle of the sight at 400 yards. Other ranges relative to the optical center will be:

100 -8.9
200 -7.2
300 -4.0
400 +0.0
500 +4.8
600 +10.3

How to find a scope's optical center before you mount it: Just put some round head Nylon screws into a base board, set the scope on the heads and rotate and tweak the adjustments until the cross-hairs stay centered on the same spot as it turns. If the screws are not rigid enough, make a wood base that lets them be screwed down firmly.

Basic concept:


Wood base version:

[stagpanther]

Very cool--is the goal of finding the optical center to then adjust the zero for "maximum coverage" of possible range elevation adjustments--and that mechanically will be "most accurate"?

[Koda94]

We need a "like" button for posts like Unclenick's.
Very useful information there.

[Unclenick]

Thanks. The purpose of the rig is to get the scope on its optical center, or optical zero, without having to fire a bunch of rounds to find it. You can then use the height above the bore line added to total drop in tables at different ranges from any exterior ballistic calculator to make initial sight adjustment estimates without firing a shot. It saves a lot of ammo live-fire trial and error shots. It also lets you decide which turret will be elevation and which will be windage on a scope with symmetric cross hairs. That trick is useful mainly for southpaws.