I think your game was awesome and played it to the end. The difficulty was on point. I liked the thrill of starting a level weapon-less and getting chased around. I had a few tense moment backed against a wall and using my last bullets at the wave of aliens coming at me.
I had a bit of a hard time understanding how which gun part was influencing the final gun build but it didn't really matter. It was fun to have different weapons types to use. My favorites were the ones with the small bullets that went through multiple enemies. Good job, seriously!