Thread: can you shoot 5.56x45 in a 223

As others have mentioned, 5.56mmx45 has longer throat, brass tends to be thicker and heavier, runs at higher pressures and tolerances might not be as tight.
The Sellior and Bellot 5.56mm brass I have is ridiculously heavy (around 10 grains more for each case) and much thicker than the likes of Federal mil spec brass.

If its factory S&B ammo, you should be okay to run it. Just take note if you reload with it the difference in pressure if you mix the brass.

Just for giggles, my bolt action .223, a Sako 75 has a much longer throat and slightly bigger head space than my semi auto .223, a Schmeisser AR15, which has real tight chamber.