One of the best shooters yet.
If people who made flash shooters did this good of a job, I think I'd play more of them. This game was absolutely amazing. I got all the medals, achievements, etc, played all the levels several times through for fun. The weapon system is brilliant, and I particularly like the ammo conservation system (which even big name shooters) tend to ignore. I just wish that it extended to the pistol.
Alas, my only problem is that in order to start a new game (on a new difficulty) without deleting your data requires you beat the final level. Which I have done exactly once... on the hardest difficulty, and was stupid enough not to... choose to change difficulties.
Also, I played this game on Armor Games, not Newgrounds, but I assume its the same here. Also you should implement Newgrounds medals, because honestly I think I'd play it again here for them.
That's all. Thanks for the amazing game, 11/.10, 5.5/5
And seriously, a zombie game with this engine would be amazing. Just saying.