Yes. It's a higher pressure version of the .223 which is why you can shoot .223 out of a barrel made for the 5.56 round...but you can't shoot a 5.56 out of a barrel strictly designed for a .223
@txradioguy @Cyber Liberty Unless it's .223 Wylde.
The primary difference between 5.56mm chambered rifles and .223 chambered rifles is the dimensions of the chamber. .223 chambered rifles have a shorter leade (distance between case mouth and start of rifling). This means the pressure spike you get on a .223 rifle firing a 223 cartridge will be bigger than of you fire the same 223 cartridge out of a 5.56 chambered rifle. And if you fire the 5.56 NATO round out of the 223 rifle, the additional powder charge combined with the shorter leade can create dangerous and damaging pressures.
223 Wylde has the same chamber angling as 5.56, but it has a tighter bore size. So you get the slightly better accuracy of 223 with the ability to safely shoot 5.56.
I bring this up because increasingly manufacturers are selling 223 Wylde rifles instead of 5.56, and if you hadn't encountered one, you might think this had the same drawbacks as 223.