It was a Sig SG 551, and chambers the 5.56 mm NATO round. I believe the one used in the movie also had the black furniture rather then gray seen on it's longer barreled parent the Sig SG 550.
Here's a picture from that scene:
http://cinema.paradiso.no/filmer/1998/ronin/bilde04.jpg
[This message has been edited by Victor Romen (edited March 26, 2000).]