Yes, the AR-15 will do you fine to 400-700yrds. Military shooters and competitive shooters stretch the AR-15 to 1000yrds. My own standard Bushmaster 16" standard AR-15 reaches to 400 comfortably (I don't have a longer range available).

One thing you MAY encounter... If you have a standard AR-15 with a fixed carry handle, your scope will be pretty high off the bore. It's not only a fairly uncomfortable and unnatural shooting position, but SOME scopes won't have enough recticle adjustment to bring the crosshairs onto targets. Also, when you mount a scope that high, you'll have an exaggerated sight/bore error. Picture the centerline of your scope and the centerline of your bore stretched out. They'll cross as an X wherever you sight in. The farther between your scope and bore, the wider your X, so the more you'll have to compensate at longer range shooting.

So go for it. Grab a glass and mount up!