Here some tips about M16 and AR-15 rifles. These have been proven by the USAR Shooting Team and written about by SFC Hubert Townsend.
Most issue, rack grade M16/M4s are two or three minute of angle (MOA) guns. That means that at 100 yards they shoot a group covering a circle of two or three inches diameter, four to six inches at 200 yards, etc. That is normal but can’t always be assumed. I was once issued a rifle that turned out to be only five MOA and didn’t discover it until shooting a five inch group at 100 yards on a paper target. No wonder I had trouble at the 300 yard line with it!
