Awesome idea... just a few problems
Falling bullets shouldn't cause any damage at all, but if they must, half damage of an actual shot should be the very most. I think it is stupid to get trapped in a corner, trying to reload, when bullets that are missing you start falling on you and you can't escape and you die early because of that.
Also, the efficiency campaign isn't self explanatory. It took a while before I realized that I lost because I lost lives (which wasn't explained until I stopped shooting to see what was really going on). Also, the speed trials seem exceedingly difficult as they require the targets to be mowed down within less than 5 seconds (in quite a few cases). This may sound like a trivial "I suck at this game" complaint, but the reason I can't stay within the desire time limit is because of slow reloads or too many targets.
This game, overall, looks great and plays great. It could easily be one of the best shooters on this site. Sadly, it has small little problems like these (and probably some others) that make it difficult to enjoy 100%. Great job, better luck next time.