The Holosun 407K X2 is the latest version of this red dot optic.  Its micro form factor is ideal for those that want to use it as a carry optic for pistols or alternatively use it as a low-profile backup sight mounted to your rifle.

Holosun HS407K X2 Reflex RDS Red Dot Sight
6 MOA Dot
Designed to work with CR1632 Battery and Battery Tray
50,000 Hours Battery Life at Setting 6
12 Brightness Setting: 10 DL and 2 NV Compatible
Miniaturized to Fit Pocket Carry Pistol
Locking Function and Motion Sensor
CNC Milled 7075 Aluminum Housing
Waterproof
Parallax-free & Unlimited Eye Relief