If there's a lot of play between the upper and lower you're going to have issues. They make a little "wedge" that helps snug up the fit between the two but a good rifle should not need one as the upper and lower "should" be a properly mated pair. Now how much of an issue is it really depends on distance and actual amount of play. Can't really give a metric on what would be a good/bad fit.
How far are we talking about shooting? What are you calling "medium range"?