Yes they are building upper receivers for 5.45x39. For example, Smith and Wesson sells them. Before about 2007 or 2008 the price of a crate of 5.45 was about $100 in the US. For that reason lots of guys in the US began buying replacement uppers and shooting 5.45x39. A buddy of mine in Arizona bought one. I've shot a bunch of it. It's less power than a .223/5.56 but the price of ammo was so good I understood his decision. I wish someone would either import a whole bunch of this ammo and some uppers or begin importing some Wolf Ammo (the made in Russia Wolf ammo). It's available for $269 a crate including shipping if you live in the US. I wish we had access to the same stuff at a reasonable price.