Customers ask what is safe? It is safe to shoot .223 Remington cartridges in any safe gun chambered for 5.56×45 mm. But also, it is not recommended and it is not safe to shoot 5.56×45 mm cartridges in a firearm chambered for .223 Rem.
When shooting .223 Rem. cartridges in a firearm chambered for 5.56×45 mm, it’s likely the shooter will lose accuracy and muzzle velocity.
What does this mean to you? If you have an AR-15 rifle chambered in 5.56×45 mm, you can shoot either .223 Remington or 5.56×45 mm safely. If your barrel’s twist rate is 1:7″ you should use bullets weighing 60 grains or heavier. If you have any rifle with a 1:12″ barrel twist you should use bullets of 60 grains or less for best accuracy. If you have a .223 Rem. rifle of any type, it is not recommended and not safe to use 5.56×45 mm ammo.