.223 Remington ammunition and 5.56x45 NATO are closely related and sometimes interchangeable cartridges depending on the firearm they are to be used with. Manufacturers typically offer guidance on this point. In general, the 5.56 NATO ammunition the military uses and is for sale to the civilian market operates at higher pressures than .223 Remington ammo. Most, but not all, AR-15s are chambered for 5.56 NATO and can fire both types of ammo.

Many loads in both calibers follow the M193 or M855 standard, both of which are military specifications. M193 ammo typically fires a 55 grain full metal jacket bullet at 3,240 feet per second, while M855 ammo fires a heavier 62 grain FMJ bullet at about 3,100 feet per second. These common standards are very helpful when sighting in your optics and you'll find both cartridges in-stock and available in cheap bulk packs here at Widener's