i ordered a Bushmaster to be safe.

It is not so much the barrel, but the chamber of the barrel that makes a difference.  There are differences between 223 and 5.56 chambers.  I think the 5.56 is a tad bit larger in the shoulder.  Someone had a great link that showed the differences.

If you are worried have the chamber checked out it may really be in 5.56 already.  And if you don't have a chromed chamber, you can get your chamber reamed out to 5.56.

.