Sightmark makes a cvr 640. It's cheaper, has lower resolution, and no sound. U can't mount this one directly to your scope or gun bc it can't handle the recoil.
The mini dvr from ultimate night vision is more expensive but records in HD and has sound. It also has an extra cord with a single switch that turns it on and starts recording or turns it off and stops recording. This one can handle the recoil of at least a 6.8... Not sure about .308