Hi Gearjammer, You can use the 5.56 brass but there are physical differences between the .223 rem and 5.56 brass. Once you find and collect a brass type you like you may want to stick with the same type during your testing. Mixing the brass will work for "out there to have fun" but you may need a different load in them for accurancy. Be careful not to start at maximum based on the books. Start with a lower combination and see what is best for your rifle. I hope this helps.
HalfTrack