I've wondered the same thing, but I've never gotten a better answer than "as long as it's not touching". I figure that can't be all there is to it, since both the scope and barrel will flex when a shot is fired. But, it seems to me there are too many factors for a single answer to apply to all situations, so I just make sure I'm comfortable with the amount of separation. That usually means that if the gap isn't at least a credit card thickness then I'm getting a higher set of rings.
I've also heard that the closer the scope is to the barrel the better, but I think that has more to do with not having to raise your hold to get the correct view through the scope.
Just a Shooter
