Just buy one - SIG SAUER Romeo 1 Pro 1x30mm Reflex Red Dot Sight

Really nice sight for the M-18. Much faster target acquisition and enhanced accuracy. For example, instead of aligning both the rear and the front sight to the target, all you need do with the red dot is place the dot where you intend to shoot. Great brightness controls, easy to adjust the dot, and built from solid materials. The dot easily mounts to the M-18 by taking the slide off, pulling off backplate and extractor assembly, and unscrewing the two sight screws underneath, removing the rear sight and top plate. The dot screws directly into the top of the slide with the screws provided.