20 MOA Mount school

[Kswhitetails]

[Crusty]

When you were zeroed with the un-canted mount did you have 20 MOA of down travel left in your adjustment?

[98Z]

Bore sight it to zero, at whatever distance you zero. Run it, fine tune it, and call it zero'd with the 20 MOA mount. What the mount does it give you more travel in your scope elevation range for longer distance shots. You're just cheating the adjustments inherent in your scope. You will end up zeroing higher in your elevation travel, so you have more drop available over longer distances.

Don't go too far into the analytics of it - just zero it at your distance.

[bj139]

Is your mount on correctly?

[Klem]

Some information missing here to know whether you have a problem or not.

All scopes have an elevation range that is fixed by the design. If everything is squared on a 0-MOA mount your boresight will be at the half-way on the elevation range. Half your range is never going to be used because you need to lift the muzzle to reach further distances down-range, not drop it. So you may as well use some of that redundant elevation with angled mounts like your 20-MOA. 20MOA is now added to the usable half of the elevation range. You may not need it if your scope has a lot of elevation to start with, or you may never need it because you're not going to shoot way out to where it's needed. But you don't sacrifice anything by having it, especially when 20MOA mounts are the same price as 0-MOA mounts.

I would now shoot that boresight, zero the rifle and see what actual elevation you have. Until you do that you are just thinking you might have a problem, and you probably don't.

[Kswhitetails]

When you were zeroed with the un-canted mount did you have 20 MOA of down travel left in your adjustment?

New mount, new optic.

[ricsmall]

Which scope do you have. I’ve mounted some scopes with a smallish elevation adjustment range that wouldn’t zero at 100. One for instance was a vx II on a Ruger Precision rimfire for my son. That rifle comes with either a 20 or 30 moa rail from factory. Had to change scopes. That’s way too much rail for a rimfire, especially with that scope

[98Z]

There are 3.4377 MOA per Mil. A 20 MOA mount is the direct equivalent of a 5.817 Mil mount. That's usually not significant enough to get outside the range of most modern scopes.

The scopes available elevation travel should not be an issue in this. Listing the exact scope would help, though. Modern scopes won't have an issue.

[centerfire]

[Kswhitetails]

Athlon ARES BTR moa. 80moa of vertical travel. The zero stop came set from factory at 2clicks (1/2 moa) from the bottom. Dialed all the way down, it appears high at 25 yards. Like Klem says, this is an academic discussion- hence the title, until I can get rounds on target.

I’m trying to figure out what to expect and making sure I properly understand the system. So far, it seems that I’ve been basically correct in my assumptions. The 20 moa in the mount (should) elevate the impact point if everything else is equal. This means that I should have to dial down 20 moa, or close, to zero the rifle set up this way. Which means I would have an extra 20 moa, or close, to use beyond zero range before running out of elevation at longer range.

The goal is to get set up to go learn how to shoot at 1k for personal satisfaction, and friendly competition. Nothing serious yet. When it gets there SH and I are gonna talk long and hard about his 6 Grendel.

[98Z]

You're overthinking it. 80moa of travel in your scope. Split it, 40 up and 40 down. Your mount will give you a 20/60 split, favoring distance.

Just go zero it, instead of mathing it out, and it'll all work out. Shoot the mount, shoot the gun. Don't math it out on the bench. Don't worry about bore-sighting it, if that's the hang up. Get out there and Shoot the gun.

[98Z]

Start at 25 yards, if necessary. Get it on paper. Go 50 after that. Zero 100, 200, whatever. It seems like you're worrying about the math - and it's not as bad as it seems. Shoot it, zero it. Don't overthink it.

[bj139]

If you can't get a good boresight at 25 yards you are not going to git er done.
For a 100 yard sight in you should hit about the scope height low at 25 yards.

[wideglidejoe]

I'll be following this, I'm thinking about doing the same thing. I've never messed with mounts with add'l elev built in, I've always concentrated on max yardage for ethical shots on game animals based on velocity/expansion and remaining ft/lbs energy available, but now thinking of longer yardage on paper/steel with the 6.5 Grendel.

I'm using the Leupold Mark AR Mod 1 4-12X40 SKU#115392. Specs on Leupold website show 67 MOA elevation, so from reading comments above, I'd wind up with 67/2=33.5-20=13.5 & 53.5?

Lots of good info available here, thanks guys, I'm enjoying it!

[Jacko]

so from reading comments above, I'd wind up with 67/2=33.5-20=13.5 & 53.5?

Pretty much correct. However, scopes are usually set to the optical center so as you said 67 range of adjustment would be 33.5 up/down. But the optic will be about 2.5-ish inches above the bore for most AR's, so if the optic is set at dead-center and 2.5" above the bore and you shoot at 100 yards the bullet itself will drop around 3 inches at 100 yards from a level barrel. You need to take the scope height and the bullet drop into consideration.

Basically your 13.5/53.5 is correct but you still need to factor in the up adjustment that will be used to zero the rifle (100 yards in this example).

It would be a bit more accurate to calculate your expected adj range this way:

67/2=33.5 (mid point) [2.5 scope height + 3.0 bullet drop = 5.5] now go back to 33.5 (of available up adjustment) and subtract 5.5 (appx. adjustment needed to zero at 100 yards) = 28.0 (of remaining up adjustment w/100 yard zero). With a zero MOA rail you will be left with 28 up remaining and 39 down. If you add a 20 MOA rail you will have 48 up remaining and 19 down.

I just ran some estimated numbers for a goof using Hornady 123gr Black ammo...with that scope (Leupold Mark AR) you should have enough elevation remaining (with a zero MOA rail) to get out to 800 yards at the crosshairs, with a 20 MOA rail you can get almost 1,100.