Check this one out...
They are fairly new and were originally made for a bow but it comes with a shotgun mount and universal rifle mount as well as the bow stabilizer mount.
A friend of mine just purchased one so that he could use it on his shotgun and bow.
Waterproof up to 30 feet and is a lot cheaper than a GoPro. ($230.00 with all mounts included)
http://tactacam.com/