Castor- not to nit pick at you- but things are easier when we all use the same terms.
A CARTRIDGE consists of a cartridge case, primer, powder, and bullet.
With the exception of muzzleloaders, all firearms have a metallic cartridge case. However, they are divided (mainly) into Brass and steel (with a few aluminum one thrown in for confusion)
Would have been easier to get an answer if the question had been "Can I shoot steel cased cartridges in my AR?" And as the guys said up there, yes, you can.
DO pay attention to caliber, tho. If your upper is marked for 5.56, then you can shoot 5.56 or .223. However, if the upper is marked .223, DO NOT shoot 5.56 ammo in your rifle.