[Guy]

I've seen them in use and people swear by them.

http://www.eotech-inc.com/images/obstruct.gif

The Magic of Holography
In holography, all the information required to reconstruct the reticle image is recorded everywhere in the Heads-Up display window. If the window is obstructed by mud, snow, rain, etc., the HWS remains fully operational , with point of aim/impact being maintained. Even in such extreme cases where the laminated window is shattered, the HWS is fully functional! As long as the operator can see through any portion of the window, the entire reticle pattern is visible on target...the operator can still engage with confidence.

Durability:

The HWS has been designed and tested to provide consistent, reliable performance even in the most hostile operational environments. It is waterproof, fogproof, shockproof, and temperature proof.
The HWS is designed to withstand Mil-Spec drop tests and still remain fully operational and hold zero. The HWS has been extensively tested in a punishing recoil simulator generating 3,500 G's of acceleration in less than 0.5 milliseconds (the recoil of a .454 Casull revolver) as well as various environmental chambers).

The Heads Up Display is constructed with a 3 layer, shatterproof laminate glass that is 3/16" thick for added durability. Additional protection of the Heads Up Display is provided with a "roll bar" ruggedized hood

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