.223 Remington, also known as 5.56 x 45 mm in military guise, is actually firing a projectile with a 0.224 diameter.

You should already find abundant stocks of .223 Remington or 5.56x45 ammunition (both should be treated as interchangeable as your rifle is chambered for 5.56).

If you are asking this question because you are considering getting into reloading, then please note that this question indicates your need to obtain and study at least one, possibly two or three reloading manuals and then read - no, study - the section at the front on "How to Reload".