barrel be made to handle 5.56 NATO and the upper made to only handle .223?
The barrel is the upper. The barrel plus the chamber and bolt carrier group, that is. Probably that was just a typo on your part and you meant to type "lower", but lets make sure you are clear on that first.
I don't have a lot of experience with 5.56 vs .223 and don't have a super amount of experience with commercially available ARs, so I can't speak of what Bushmaster has a tendency to do. My best guess is the rifle did not come as a complete rifle, but was pieced together and then sold as a single weapon. If this is the case, we would need to know more specifics on the upper.
I could be completely wrong.. I'm just going off the link I showed you earlier that it looks like the lower is purchased separately. If I am wrong, hopefully someone will be here shortly to correct me.
I feel like all I've done is repeat my first post with different words.. didn't mean to be repetitive!