The night vision-compatible Meprolight M21 reflex sight is powered by ambient light through a fiber optic system and by tritium in total darkness. Whether you choose a bullseye, triangle, or dot reticle, the brightness self-adjusts to any light condition. With no switches, electronics, batteries, or internal moving parts, the M21 is unaffected by water and runs in battery-killing cold or desert heat. The Meprolight M21 is always on and always ready. It's available from The Mako Group, exclusive distributor of Meprolight combat optics.