Pressing I to go back to the inventory DOES NOT work. Also, the game froze when I paused. It froze and would not let me unpause, yet everything else on the computer worked fine.
Good concept, but it got way too easy, way too fast. Once you have all your upgrades, the enemy is too easy. Should have added different types of enemies, i.e. helicopters, APCs, and so on, as well as more weapon upgrades, rather than just machine gun. 3/5 for concept and potential.