556 and 223 are interchangeable in most rifles. Same with 7.62 and 308. So what if the military specs call for slight differences. Look at a loading manual sometime. There are a wide range of different specs for every chambering in the books.
At worst you might find a load that may not cycle reliably in some semi-autos. You will find the same for 30-06, 270, or any other chambering. A box of Remington 270's may be loaded much differently than a box made by Winchester. Including minor differences in the chambers of the rifles.