To Cant or Not to Cant - Precision Optic Mounts and Rings

[Gutshot John]

I understand the general principle of a 20 or 40 moa cant to an optic mount as necessary to achieve accuracy especially at greater ranges but can someone explain the ballistics a bit? When is it necessary? When is it not?

Similarly, why choose one or the other (or none at all)?

Since there are several choices on the market, I presume that there are different requirements for each selection.

What does Badger for instance bring to the table that Warne, Brownells or others don't?

[STJ]

I wouldn't say it has anyting to do with accuracy...

The canted bases are for long range shooting. If your scope only hase 50moa of vertical adjustment and your round drops more than 50moa at say 1000 yards, you can't dial in the height needed to allow you to put the cross hairs on the target. If you use a 20moa base, that adjusts the scope up 20moa to allow you to center the crosshairs.

[sinister]

If your base has no forward slope (say on a .308 caliber Remington 700) and you're using a scope like a Leupold M3LR you're going to run out of "Up" elevation around 850 yards. You will then need to use your mil-dots to hold over (instead of just dialing up a few more minutes to get to 900 and 1,000 yards).

Most folks will never shoot that far and will never need slope correction.

[Gutshot John]

So with a lot of adjustment range (~100moa) it's not really important?

[MAX100]

If your scope has 100 moa total elevation adj. After mounting on a standard base that gives theoretically 50 up and 50 down. When you mount a scope on a 20 moa base it angles the scope downwards and takes about 20 moa off the bottom and puts it up top. You now have 70 moa elevation up and 30 moa down.


If your scope only has 50 moa total adj and you use a 20 moa base it is possible that you could run out of adj on the bottom before you can zero @ 100 yrds because you will only have about 5 moa on the bottom to play with.

I would use a moa base because they don't usually cost that much more and why waste the adj on the bottom. A Scopes turret adjustments are known to be more accurate if you don't stretch them to their outer adjustment limits trying to reach your target.

With a 308 you need around 50 moa of elevation up adj to be on the safe side to reach 1000 yrds.


EWG 20 moa base & Burris XTR rings is a very good lower cost combo.



GC

[sinister]

If your mount or rail holes are off true even a smidge and you have to eat up any windage correction to zero (and you don't have a sloped base) your erector tube can also bind up against the inside of your scope at the top of your travel (say trying to hit at 900 to 1,000 yards with a Leupold 3-9X on a flat base).

No matter how many clicks you try putting on to correct for a full-value side wind it won't matter if there's no range-of-travel room for the internals to go. You're then stuck with Kentucky windage again.

[Gutshot John]

What are the implications for using an offset RDS and a sloped rail? Since I'd imagine the main optic would be used for anything 100 yards and beyond, I don't see that it would make much difference inside of a hundred, but I could be wrong.

[MAX100]

It should work. You really don't need a RD sight on a LR rig.


GC

[Gutshot John]

It should work. You really don't need a RD sight on a LR rig.
GC

"Need" doesn't really have much to do with it, strictly speaking I don't really "need" a LR rig either.

I'm probably going with a fixed 10x so it would be nice to have an offset RDS for shooting inside of 100 yards. I hate the idea of a piggyback so the offset mount would be the only option I'd consider and so I wanted to be sure there wasn't going to be an issue with a sloped base.