…Almost interchangeably. .223 Rem rifles have a shorter freebore than 5.56x45mm NATO rifles. That is, the distance from the mouth of the case to where the rifling in the barrel starts is going to be significantly shorter on a .223 Rem barrel. The result is that you can, maybe, possibly, develop a much higher pressure in a rifle marked for .223 Rem than you would for 5.56x45mm NATO, since the resistance to the bullet is going to start somewhat sooner in the powder burn cycle.
What this means is that you should never use M193 or M855 ammunition in a rifle marked for .223 Rem.
Ideally, if you were loading ammunition for a bolt action rifle, you would measure the freebore, and load your bullets so that there was no jump to the lands. That is, when the bullet was chambered, you would want the projectile already in contact with the lands, so that there was no ‘jump’. But the freebore is so significant on 5.56x45mm NATO that if you load ammunition such that there’s no jump, or only .020" jump, then your bullets are too long to fit in your magazine.
I think that you have similar issues with .308 Win and 7.62x51mm NATO, but I’m not sure off the top of my head.
TIL that .223 Remington and 5.56x45mm NATO are very similar but not identical cartridges. Weird!
They aren’t identical but the cartridge is so similar they can be used interchangeably.
This is dangerous.
TL;DR:
You can use .223 Remington in a rifle built for 5.56x45
You should never use 5.56x45 in a rifle chambered for .223 Remington
Are you worried about the pressure? The round will absolutely feed.
Yes, of course I’m worried about the pressure.
You can also feed and fire a .300 blackout in a 5.56 rifle/barrel, it will just destroy the weapon, and maybe your body if you’re not lucky.
Edit: https://youtu.be/RbfIkaNlECo?t=127
<nerdery>
…Almost interchangeably. .223 Rem rifles have a shorter freebore than 5.56x45mm NATO rifles. That is, the distance from the mouth of the case to where the rifling in the barrel starts is going to be significantly shorter on a .223 Rem barrel. The result is that you can, maybe, possibly, develop a much higher pressure in a rifle marked for .223 Rem than you would for 5.56x45mm NATO, since the resistance to the bullet is going to start somewhat sooner in the powder burn cycle.
What this means is that you should never use M193 or M855 ammunition in a rifle marked for .223 Rem.
Ideally, if you were loading ammunition for a bolt action rifle, you would measure the freebore, and load your bullets so that there was no jump to the lands. That is, when the bullet was chambered, you would want the projectile already in contact with the lands, so that there was no ‘jump’. But the freebore is so significant on 5.56x45mm NATO that if you load ammunition such that there’s no jump, or only .020" jump, then your bullets are too long to fit in your magazine.
I think that you have similar issues with .308 Win and 7.62x51mm NATO, but I’m not sure off the top of my head.
</nerdery>