im currently doing a build where ive stolen the cameras from small portable HD devices, and mounted one to my helmet and one to my rifle, working on the wiring so that i can view the front cam via cinima glasses which im going to mount inside of the helmet.
if your plan is to upload the footage, id say mounting on the end of your rifle wouldnt be to grate, you would only be left with a limited amount of footage where the camera isnt pointed at the ground, but the footage you would get would be nice, so mounting it on your helmet would be better, but id also then say would be better of mounting it on your off side.
so if you shoot a rifle in your right shoulder, mount the camera on the left side of the helmet. you get a clearer view and proves most popular with people