I did this a while back in Vegas - it was a fairly quick job so wasn't perfect but did illustrate the effect:
First, prepare your media:
1) Gun Barrel Shot with alpha channel in the centre where the actor stands (I used: http://www.duffzone.org/desktops/alex/bond/1024x768.jpg and then cut out the middle)
2) Blood - now if you've got a way of making real blood drops that slide down the screen at different rates - (e.g. special effects software) - then use it. But for me I opted for a much quicker job of making a red shape with a wavy-drip-shaped bottom - coloured deep red with a slight bevel. This was then moved down the screen at the end of the shot.
First film the actor in the centre of the screen walking along and move with them so they stay in the centre. Then have them stop and fire at the camera.
Then in Vegas overlay the gun barrel shot so it appears the actor is walking inside the gun barrel (this may take practice to get the zoom right.
When he fires, add a gun shot and have the right blood image slide from top to the bottom - perhaps using motion options to give it an uneasy fall rather than smooth. At the end pan the gun barrel track - similar to the real effect (just use you tube to find many examples!) - and add some music to complete the effect.
Like I say, it's been a while since I did this but hopefully that will give you a few ideas!?