I use two Hero 3s, a black and a silver. One I mount on the NVG mount on my FAST helmet and the other is for my gun; either using a handlebar mount on my stock or a neat little aluminium RIS mount If I want to rail mount it.
I've got 2 action cameras that mount on my helmet but can just cable tie them to my rifle under my aimpoint or mount them under the rail with my old aimpoint mount (I bought a new flip to side mount)