In my experience, by the time you get a cheap spotting scope to 40x, the mirage effect is so bad on most days that it's useless. The other problem is that a good quality scope that'll spot .22 holes at 300 yards is going to be expensive.
I did see a home-brew rig that worked really well for one guy at our local range. He had a remote TV camera in a heavy steel box, tapered at the rear in case some idiot shot it. He set this about 15 feet from the target and focused it on the target, then he just checked his shots on a portable TV at the bench. It was a pretty slick idea, and it worked really well.
