For other variants, see 552 Commando and 556xi
The SIG SG 550 is an assault rifle manufactured by Swiss Arms AG that chambers the 5.56x45mm NATO round.
The SIG SG 550 is a weapon that was supposed to be featured in both Tom Clancy's Rainbow Six: Vegas and Tom Clancy's Rainbow Six: Vegas 2, but was later cut from the final product. This weapon was found among the files of the PC versions of both games. There seems to be a Rifle Scope on it, so the weapon is presumed to be the SIG SG 550 Sniper.
The SIG SG 550 is seen as the third weapon in the third column from left to right.