The systems which has worked great with me is made by Adrenaline Systems.
It comes with everything for mounting on the Gun and Head:

GUN CAM PRO- TACTICAL RIFLE 580 VARIFOCAL SONY CAMERA ASSAULT HELMET CAM AdrenalineCamera.com



I also got their Handheld DVR which is super light and am able to take great stuff on SD Card. This is the DVR 200:



You can now get a combo kit so do you research. They seem to always have great sales.