Somebody with an assault rifle will have shot a shotgunner long before they are close enough for the shotgun dude to do anything. Unless the map is nothing but rooms and corridors. However, since they don't know beforehand the turf, it's a safer bet to take a rifle. Sinon is even betting on a sniper rifle and a simple pistol for a side arm. But then again, she was looking at a target 10 meters away through the sniper rifle scope, so I guess in this game nothing matters.