For tight groups, you'd ideally be talking about a $2K AR. Something along the lines of a $400 lower (match trigger), $1000 upper (maybe an 18" 1:8 stainless match barrel), and then $600 for optics...plus heavier match ammo and a fair amount of practice. At that price (and range) an AR-10 or M1A starts to look like the better option.
If you're just talking about tapping a 12-18" gong for fun at that range, a typical carbine/midlength could suffice if wind isn't a huge factor.
I would sure struggle to group at that range, but my longer range skill set is pretty limited. Plinking at a decent sized gong is something different entirely, however.