There is enough difference in performance between the 5.56 and .223 to warrant some consideration if looking at the two for a specific purpose. In some cases, you will want to be very picky about the type of round you are shooting based on your desired external and terminal ballistic performance.
Typically the barrels for each caliber is designed to cater only that specific loading and using the 5.56 and .223 interchangeably in a .223 specific barrel can cause excessive wear and possible damage. But in most cases, a little bit of 5.56 wont hurt your rifle, just like how a little +P in your pistols isn’t going to hurt them. Based on your rifle, your needs for pressures and loadings, in general, may vary greatly. A 20″ barrel will have different flexibilities when compared to a 10.5″ barrel. Just keep that noted in the back of your head if you are weighing those 2 options. Personally, even though I have a 5.56 barrel, I like to shoot .223 ammo when I operate operationally on my range in my backyard, so I don’t think I’d really care if my rifle took 5.56 or not. It is inexpensive and is easy to shoot, so I will shoot .223 all day, any day. But my rifle (IWI Tavor) was recommended to have a 5000 round break in with 62GR 5.56 ammo due to the tight springs. That could be a big thing to consider if you have a new rifle and need to break it in. The higher pressures may just be what the doctor ordered when breaking in a rifle quickly and properly.
The biggest difference between what the 5.56 and .223 rounds offer is external and terminal performance based on velocity. Now if loaded the same with identical pressures and projectiles, the differences would be practically unnoticeable for the most part. But as the market stands, not everyone loads their own ammo and it is up to the manufacturers to load their ammo to whatever loadings they feel is safe but still practical. The .223 has a wider variety and flexibility when it comes to soft point and hollowpoint. There are a few companies that load soft point projectiles to 5.56 pressures with success, such as Speer. You may find that most 5.56 loadings consist of heavier weight bullets, while .223 being a varmint round primarily by design, will tend to cater to lighter loads.
