Well, I've never seen "Grosse Point Blank", or "Shawn of the Dead", or much of Monty Python, so I was completely in the dark here, but I enjoyed reading this, even though I'm not sure about assassin!Harry. I kept reading to the end, (but I did have to check that Snape and Harry were still alive first!) and think you succeeded in providing a rollicking good "gangster"-type story.
I liked your Ron and Hermione here; it was nice to see that Ron turned out to be reasonably competent.
And I very much liked that Snape managed to save himself. I thought the imagery of him handling and reloading the gun was quite erotic. ;-)