Lucky Gunner has a look at .223/5.56mm ammo.
They tested Federal (brass case), Wolf, Tula, (both steel case, polymer coated) and Brown Bear (steel case, lacquer coated) ammo.
Executive summary: Federal was the most reliable, the most accurate, and the dirtiest(!). Tula was absolutely unreliable in the Bushmaster carbines originally used, but worked in other brands.
But, really, you need to read the whole thing...