Two scopes in the $400 price range that I have had great experience with are the Vortex Viper 6.5-20X50 PA. The Apex XP 6-24X50 from Alpen has a unlimited lifetime warranty for any owner. I only have a few hundred rounds of 7.62 on my Apex XP but so far it's as nice as the Vortex in optic quality and features.
http://alpenoptics.com/mm5/merchant....duct_Code=4058