ive shot it in my 5.56 but if its for a AR just remember that if the gun is marked for 5.56 its ok to shoot 5.56 or .223. and if the gun is marked for .223 only shoot .223, because its a heavier grain and can blow out the seals in the gas chamber, "i seen this happen at a gun range" it doesent really mess it up just makes u have to get it serviced, but other than that u just gotta make sure u clean ur gun good after any shooting