AR-15s are chambered for 5.56mm x 45 NATO ammunition or .223 Remington. They are not always interchangeable. In general, barrels chambered for 5.56mm ammo can reliably use .223 Remington but the opposite is not always true. Also, There are many different types and weights of bullets available from lightweight 43 grain varmint rounds to 62 grain armor piercing military rounds. Some rifles cycle better with a particular type of ammunition. Also, some rifles are more accurate with a particular typ of ammunition. The best way to know which ammunition works best with your rifle is to buy a small quantity of different types and go shooting! Keep careful track of targets and which rounds go with it. Record all of your data in a table format. Since the AR-15 is a centerfire round, re-loading is a viable option and gives you a wide variety of choices. You can vary the amount of powder, type of powder, bullet weight, bullet type, etc.