...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.

[–] [email protected] 0 points 5 days ago

There's too many stories of people successfully doing it for it to be impossible. At most I'd say you need to be aware of your rifle. Most reputable ones will be fine though. If you buy a 100 special that's on you.