The actual M4 used by the Army fires a 5.56mm round. That converts to a .223 caliber, better known as the .22. Currently my friend and I believe that the magazine in the weapon either fires 9mm or .45. Yes, they have conversion kits for the M4 and its variants. But the target has impact holes that look like they came from a 5.56 or a .22. Anyway, checkout cav arms and the trou is muffin topping anyway, so I could give a fuck less.