Ronaldo Olive dug up another interesting story for us. Back int he late 1980s, Argentine weapon manufacturer Fábrica Militar de Armas Porátiles Domingo Matheu decided to create a FN FAL chambered in 5.56mm. They strangely enough decided to adapt the 5.56mm Steyr AUG magazines to fit into the modified FAL.
Following cancellation of its native FAA 81/FARA 83 rifle program in the late 1980s, Argentina tried another way to get its Army equipped with a 5.56 x 45 mm rifle in the early 1990s. This consisted of adapting the 7.62 x 51 mm FN FAL (which was then in full production at Fábrica Militar de Armas Porátiles Domingo Matheu, in Mendoza) to fire the smaller round, at least one prototype having been completed. This used the 30-round Steyr AUG plastic magazine with a steel interface for adequate insertion into the FAL mag housing. The program did not proceed much further, though. I was lucky enough to fire this unique rifle at FMAP-DM in February 1990.
Many thanks to Rolando for sharing this interesting story and photos.