Looks like the box says 5.56x45 so I would assume NATOThis should be fun..... is this a NATO round or no?
So, here is the FYI (based on your title)
.223 = not NATO
5.56 = NATO (and sometimes not)
difference in charge and FPS
You sir, have 5.56, not .223 = although the projectile size is the same (for the most part)
Most .223 can't chamber a 5.56 because of the pressures. However a 5.56 can chamber a .223, depending on the twist.
I do apologize - I could not help it.... (been a rough day....)