Overall
well the concept is good, but the design is flawed, I can get my weapon out and start shooting (I guess I did better than most) but I can never kill enough in time, maybe not starting the time until you get the weapon out or something, that might help, the time needs to be a bit slower as well. Maybe you could fix these things, if so it maybe be quite nice.
My rating is a bit generous, but I'm giving points for the overall effort of the game, because with a bit of tweaks to the game, it could be very nice.