The only thing I can suggest is to keep a count of ammo used, which you could do by modifying the ammocount script above. You would have to allow for picking up more ammo, changing half-empty magazines and dropping magazines as well as changing weapons (with an immediate change in ammo amount). It is a very challenging scripting proposition you have there.. (sound fun yet?)
Then, add a 'hit' eventhander to each person. You'd have to play with this a bit, I don't know whether it passes the shooter - I think it does. So just increment a value for each player.
So, with those modifications to ammocount, you now have a script that counts shots fired and a script that counts hits, and again, 1 + 1 = 3.
There is also a script somewhere in the ed depot which was pre-event handler and could keep track of who killed who, but I don't know how it did it as I never examined it.
I have never seen a script that will achieve exactly what you want.
Try the Browse Snippets and use the Search feature.
Cheers,
CareyBear