Sneaky makes a good point. Some airguns, like the highly popular RWS 34 like my brother has is notorious for "barrel droop", making necessary an adjustable mount or the use of shims. Oddly, my brother mounted his scope right up and never had a problem with barrel droop.We don't have a clue why.Also, apparantly all adjustable mounts are not created equal.And...you may not even need one.
Again, call Craig and Kevin @ straightshooters.com. I bought my Beeman R9 from them, and they've been more than helpful with my numerous bonehead questions and issues.