The Duke will be able to upgrade more than the base stats. Use the currency you earn from, killing enemies, exploring and looting the environment, you will not only be able to purchase new weapons, but also be able to upgrade your weapons. Of course, there are standard upgrade options for each weapon, but you can also purchase modifications for certain weapons, such as a cheek rest for a rifle to decrease sway, or a trigger for a shotgun to increase its rate of fire, or a larger magazine for a pistol to increase its clip capacity.
