Quoted:
Pardon my ignorance, but what is the purpose behind the cantilever spacer?
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Dot/Reflex-type optics were designed to be mounted forward on a gun, not back over the receiver like you have to mount most magnified scopes that have limited eye relief. But unless you have a quality free-float railed handguard, or you have a carry-handle receiver with a gooseneck mount, then you're stuck mounting the optic on the flattop. This leaves the optic too close to your eye, and you tend to use it more like a conventional scope. This is bad because you tend to "tunnel in" on the scope and forget to look around at the rest of the world, and you lose situational awareness and are much slower getting on target.
The cantilever mounts give you an extra 1.5-2" or so of distance from your eye, greatly improving the usefulness of the optic. It's easier to use with both eyes open, and *with* both eyes open, you can easily ignore the optic housing while still seeing the dot, as your brain keeps the important stuff and filters out the rest. You maintain your situational awareness and full field of view while still being able to pin-point your target with the "Dot Of Death."
These mounts are plenty robust enough that brush getting caught is not an issue.
-Troy