One important element that is often missed, after the scope and mounts are selected, is to get the mounts installed and aligned properly. There is a lot of good advice here concerning the type of mount you might try, and I certainly agree with most of it. I'm a big fan and the picatinny set up because it's a stiff straight line with lots of adjustment along its length. I use a 20MOA picatinny on my 6BR to help me make better use of the equipment beyond 800 yards with my Leupold 6.5x20x40.
It's rare, perhaps even unheard of, to find mounts that fit the receiver precisely. When you install those mounts, before you start tightening things down, make sure the base fits the receiver perfectly. There are several methods to ensure the fit; my personal favorite is the JB Weld bedding technique. You can check those processes out yourself. It's worth the time to get it right from the start and avoid the frustration of trying to figure out why things aren't as precise as you want them to be later on at the range or in the field.
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Hunter Safety Instructor - Rifle/Pistol Marksmanship Instructor