will this gun shoot accurate at 400-700 yards? thinking of putting a scope on it.

A .223 is hardly a "good" 600-700yd cartridge, and a semi-auto is far from the ideal platform to attempt it. Perhaps it will work out ok, but there are better options on both counts.

without a doubt that ar will take you out to 600 if you know how to get there. a good scope will really help imo so dont hesitate to go out and get a leupold or something equal. ballistic charts help to but are almost nothing without a good personal weather device. if you already have a scope on it, try just gettign out to 500 yards and keeping the gun sighted for that range and start practicing. if you can hold a group your good to go out and get better optics and all the fancy toys to go with it.

how about 300-400 yards? i have the ar-15 already

if your good enough 400 can be done with open sights. i know it because ive shot a c7 (canadian ar) out to 400 meters with very good results

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!