Let me solve your problem, get the 18", lol. I have a RRA 18" Varmint. Just FYI, it took about 300 rounds for it to settle in. I was getting about 1.5 - 2.0 inch groups and was starting to get concerned and all of a sudden they dropped to like .5 to .75 inch. I have loads worked up but haven't had a chance to shoot them yet. RRAs are very nice but I would seriously consider a custom for the same money. If you can't wait though, can't go wrong with a RRA. MOA stands for minute of angle, one inch at 100yds, two inches at 200yds and so on, that's the simple answer anyway. Good luck, you'll love whatever you get...