Another great game.
The basic idea behind the upgrading system is that you can improve your guns and even your armor of which you had none in the first. The guns are improved by means of use. If you use the gun enough it upgrades to the next level and usually does more damage. However, the armor is upgraded by purchasing the next level and those can get to be pretty expensive.
The graphics are nice, the game-play is fun and the sound is just plain satisfying.
Overall just a very fun game with plenty of replay value.
Sorry this review is so short.