Hi, I have the M&P 15T with an EoTech 511.65A holosite. After sighting it in, I have no problem putting the red dot on targets out to 500yds and hitting the spot.
There is also a scope by leupold (sp?) specifically made for the AR-15. I forget what exactly it is called, but its designed with the .223 round in mind so it takes the guess work out of targeting. A buddy has one on his and its very nice. It was around $300.
Hope that helps. Have fun with the rifle.