This is a Surefire Weaponlight known as the M300 Scout, model M300C-Z68-BK.
The M300 Mini Scout is a powerful" lightweight weapon-mounted light-powered by a single 123A lithium battery-that features a virtually indestructible LED" which generates 500 lumens of light focused by a TIR lens into a high-intensity beam with plenty of reach for close- to mid-range operations. The rugged M300 easily and securely attaches to MIL-STD-1913 Picatinny rails via its integral thumbscrew clamp and is activated by clicking its pushbutton tail cap switch or pressing its pressure-activated tape switch" included on appropriate models. Machined from lightweight aerospace aluminum with a Mil-Spec hard anodized finish and O-ring sealed to keep out the elements" the ultra-compact M300 will withstand the rigors of combat without weighing down your weapon.
Features:
Virtually indestructible ultra-high-output
TIR lens
Thumbscrew mount attaches quickly & securely to MILSTD-1913 rails