The only issue I had was knowing whether the weapon I was picking up would replace my back weapon, or my main weapon. I never really wanted the shotgun, so I'd try to swap it for the rail or machine gun, and it seemed inconsistent as to which of my weapons would get replaced.
Other than that, the game is really well polished and was fun to play. It was cool to see a Game Jam game that actually had an ending to it. I liked your final message too. Good job!