Other side and forward between the A post and the flash hider. I have never seen a barrel that was not marked except my Bushy 11.5" upper but I have never had any RRA guns or parts. My Bushy 11.5" upper is 5.56 Nato they state that when you buy it.

If it is not marked I would call them and ask how they chamber their barrels.

Here is a pic of a colt I have that is marked like 90% of the uppers I have seen.