On this note though Maarten, there were some short barrel Carbines used by SOG, but not the short barrel ones that we have in game.
The "usual" ones we use in game are the XM-177E2A model, with the extended flash hider that makes the weapon sound more AK-like, and also has the lug to mount the M-203 grenade launcher (not the XM-148 we have in game).
The short barrel one that is not in game is the XM-177E1 model, like in the attachment below. This model of Carbine was also used by SOG, likely before the XM-177E2A version was fielded.