Assuming 5.56 was in use first, why did Remington develop its .223?
The 5.56 wasn’t in use first. They both came from the same design. Armalite was experimenting with the Remington .222 but it didn’t have enough penetration for what they were looking for, so Armalite asked Remington to develop what they originally called the .222 special. This caused a bit of confusion with all of the other .222 versions out there so they renamed it the Remington .223. The original intent was to release the 5.56 as a military cartridge and the .223 as a commercial sporting cartridge, with the 5.56 firing a slightly longer and heavier bullet. The design of the 5.56 ended up being tweaked a bit as a result, and the cartridges ended up not being as nearly identical as first envisioned.
The 5.56 did end up being released first, and the commercial .223 came shortly afterwards.
and on this it’s important to note that a firearm can be chambered to accept both (AR-pattern rifles and the Mini 14 are) but that doesn’t mean you can assume one is. If a particular one says specifically “.223 Remington” on the barrel, you probably don’t want to use 5.56x45 NATO.
Certainly **do not **want to use 5.56x45 NATO in a .223 Remington barrel.
You can use .223 Remington in a 5.56 barrel. It has to do with not only the powder load of 5.56 which produces higher pressures, but also the distance between the bullet and the rifling of the barrel. A .223 barrel has the rifling closer to the bullet, and if one uses 5.56 ammo the pressure in the chamber goes above the specified value and can lead to failure of the barrel.
There is a .223 Wylde barrel that is more accurate for 5.56 NATO rounds, and equally accurate with .223 Rem. Lots of competition shooters using the AR platform run .223 Wylde chambering (though they are likely loading for .223 Remington pressures)