Most AR15 rifles will shoot 5.56, as long as the barrels are stamped 5.56 NATO or somthing like that. Many lower receivers are also stamped with 5.56 NATO or .223 Rem or somthing like this, but this is NOT what you should go by in selecting ammunition to fire. Look at the barrel. Because of the modular construction of the AR15, a used gun may have non factory upper.
Bushmaster has both a 223 and 5.56 stamp on the lower receiver, but only 5.56 NATO on the barrel.
Most Varmint guns and other non-AR15 rifles are chambered for .223, and the barrel stamping should reflect this.
Use .223 in a .223 barrel only, but use either .223 or 5.56 in a 5.56 barrel.
.223 is NOT mil-spec, but can be shot through mil-spec chambers.
5.56 brass is not stamped with .223, as it is a different size cartridge.
Shooting 5.56 out of a .223 stamped barrel can be dangerous!