How to figure the correct MOA angled scope mount base....

[Hellbender]

These questions seem to come up a lot with the rise of people shooting .22LR out to 200, 300, 400+ yards....

Do I need a 0 MOA, 20, 30, or 40 MOA scope mount base ???
Can I still get a 25 or 50 yard zero???
How far will I be able to dial with XX scope and XX MOA base???

This depends on 4 main factors:

1. Amount of available travel in your scope (if you want to dial elevation instead of holding over)

2. Ballistics of the ammo you shoot

3. Distance you want to be able to dial without holding over (depending on your scope & mount choice).

4. Close range zero you need

FOR AN EXAMPLE:

#1... SCOPE is a Vortex Viper PST Gen II 5-25x50 FFP, this scope is available in both MIL and MOA versions, it has available elevation travel of 20 MIL or 70 MOA. I never want to use ALL the available elevation adjustment of the scope (odd stuff can happen if you do, I don't want to go into detail here for clarity), I like to stay at least 3 MIL or 10 MOA from the bottom for zero (the lower the quality of the scope the further I want to stay away). So we have 17 MIL or 60 MOA available for dialing.

#2... AMMO, I'm going to use Lapua Center-X standard velocity (1080 fps), 40 grain ammo. You need to use a ballistic calculator and plug in your ammo's data (I won't go into this in detail as that is a separate topic). So with 17 MIL or 60 MOA available (from SCOPE above) I have a MAXIMUM distance I can dial of approx. 350-360 yards IF I can get the EXACT MOA base I need.

#3... DISTANCE, I want to try and maximize the distance I can dial while being able to have a...

#4... ZERO @ 50 yards (25 is usually same +- .2 mils or .5 MOA)


So with THIS example....

Take 1/2 of your scopes available adjustment, ie.... 20 MIL or 70 MOA divided by 2 = 10 MIL or 35 MOA, then subtract 3 MIL or 10 MOA (for safety) so that leaves us needing a 7 MIL or 25 MOA base.

We're going to round off the MIL/MOA conversion on scope mounting bases for simplicity...

0 MOA base > 0 MIL
20 MOA base > 6 MIL
25 MOA base > 7.5 MIL
30 MOA base > 9 MIL
35 MOA base > 10.5 MIL
40 MOA base > 12 MIL

A 25 MOA base is a little uncommon, most are 0, 20, 30 & 40 MOA.

IF you go with a 30 MOA (9 MIL) base you MAY or MAY NOT be able to zero properly at 25 or 50 yards, it will be very close with the chosen scope.

A 20 MOA (6 MIL) would be safe, BUT you will lose a little UP adjustment compared to a 25 MOA base.


If you want to be able to dial further you will have to either:

A. Buy a scope with more adjustment (a larger adjustment range would also allow a larger MOA angled base for even more UP adjustment)

B. Change your zero to a further distance and use a scope mount base with more MOA angle.


SO....TO SIMPLIFY.... if you want to maximize your dialing distance AND have a reliable 25 or 50 yard zero:

Take your scopes total elevation adjustment range and divide it by 2, then subtract either 3 MIL or 10 MOA, that is the MAXIMUM angled scope mount base you can SAFELY and RELIABLY have a 25 or 50 yard zero.

Scope ring HEIGHT makes almost no difference, except at very close ranges (under 10-15 yards or so).


DISCLAIMER: A gun or scope/mount/rings with mechanical issues or other problems may need more than 5 inches (3 MIL or 10 MOA) of adjustment at 25 or 50 yards to zero. If it's questionable, add to the 3 MIL/10 MOA subtraction above.


Hope this helps clear up some of the questions and confusion about this issue.

 

[6.5SH]

@Rob01 or another mod, can this post be stickied? Super useful info that is frequently asked about.

 

[nockhunter]

Great work!!! Thanks for your time and effort.

Mike

 

[jps2486]

Personally, I would get a 20 MOA mount then get a set of adjustable rings. Burris Signature rings work well and you can adjust them += 40MOA.

 

[Coopsdaddy]

Great work and simple to understand, thanks!