i am building an AR that was origonally going to be an m4 with heads up optics, but dew to some recent changes i have decided to use it for target shooting. i will not be competing at this time wut will be shooting alot at 300-500 yards. I am on a budget and don't want to have a Hubble Telescoping sitting on top of a 16-18 barrel. i don't really have any idea where to start 3x, 4x, ...help!