Not too long ago, the Air Force had to retrofit a LARGE NUMBER of A1s to A2s. They didn't get money to buy new A2s for such operations as Basic Training, so they had to upgrade the rifles themselves. When I qualified about two years ago, the rifle I used had been retrofitted-it had the "M16A1" changed to "M16A2" (kind of sloppy), and burst parts installed in the lower, with an A2 upper on it.
Farther back, the Air Force converted M16s to A1s as they were needed. I have seen XMs in the field that looked like they had been issued new in Viet Nam (I ain't THAT old!), prong flash hiders and slick sided uppers, all sorts of variations. Air Force Armorers use the parts they can get to make the rifles work.
The bottom line: the Air Force gets "hind tit" when it comes to combat weapons most of the time, so it is most likely that they surplussed a lot of 20" barrels when they had to make up a lot of M4s. That would go for spares, as well.
PS.: It would be a very nice thing if the Air Force actually had enough rifles (A2s, M4s, whatever) for people in the field in Southwest Asia to actually have them. Not gonna happen until the Army and Marines get what they need. Sigh. Ok, rant over.