If stamped .223 Rem. you can only shoot .223,no 5.56.The reason is two-fold,higher pressure loads and 5.56 has a longer chamber throat than .223 and both can cause high pressures in .223 chambers into the 70something range. You can reload military brass to loadbook specs starting at minimum and working to max and watching for pressure signs. I would stay 1gr below max.You must remove primer crimp from primer pocket before attemting to seat primer. ,,,sam.