Personally I would feel comfortable taking a 300 yard shot with a red dot and magnifier. Could it be done with just a red dot or iron sights, probably. Would I make that my first choice no. I like the flexability of an open feild of view at close range and the ability to go longer if I need to.
Isnt 5.56 ballistics somewhere near 36/310 yards for zeros?
Depending on hunting conditions I would probably use .308 for the 300 yard range.