Re the FN barrel thing, FNs first government contract was for A1 barrels used as replacements. When the A1 type was dropped, the barrels were surped out.
Under the Clinton administration, they were supposed to have been demilled, and sold as scrap, but a fair number of them appaently just went out in one piece.
More recently, we've been seeing A2 barrels from FN on the market, for the same reason. The Army has gone to the M4 type, A2 barrels are superfluous, and are being sold off.. Apparently it hasn'r occured to the Army Ordnance people to ship them to the Marines.
I would hazard a guess that you're going to see some A2 type uppers hit the market, now that the USMC has gone to the flat top uppers on their rifles; just like there were a bunch of Colt A2 uppers around a few years back, when the Army went to the flat top.