6 MM, I have found with my X sight that it will not group as good as my day scope but I have no trouble hitting coyotes at 200 yards plus. I put a mark on the focus knob where it is focused best after dark so I can put it back easily also it has to be focused in a different place for day use. I got a butler creek scope cover and drilled a hole in it so I don't have to keep putting that cover off and on. I also when out calling I leave my scope on and let it go to sleep and when I see a coyote I hit the record button for the RAV and it wakes it up and starts the RAV, that record button you have to push it fast and let up fast or it don't like to work. I have had 2 of the x sights for about a year now and am pretty happy with them, they are not perfect but pretty good, also with the stock IR if you don't take the battery's out it will slowly drain them.