The 5.56x45mm cartridge, also known as “5.56 NATO” containing the 55 grain full metal jacket (FMJ) projectile are by far, one of the most widely used rounds for the AR-15 platform. This full metal jacket can only be fired being in barrels labeled as 5.56x45mm NATO. The 55 grain projectile (.224 diameter) is the ideal weight for a wide range of barrel twist rates to include the 1-in-7, 1-in-9, and 1-in-8.
MIXED HEADSTAMPS! Your ammunition may have a mix of 5.56 and .223 headstamps.