I have a Nikon Prostaff 3-9x40 on my AR-15, with a Burris P.E.P.R mount. I can easily smack the steel plate at 300 yards at my local range. Plus, the scope is well under 200 bucks and is the best rated in that price range. I have the same scope on my hunting rifle, and it's helped me take two elk in two years.
IMHO, you can't do much better than a Nikon, no matter what it's on.