From all of the research I have pulled in online, the only thing that I could find was this:
There is a bonus score multiplier of 8% for each successive kill you make. This is reset if you miss but not after an amount of time has elapsed. It also doesn't matter if its a headshot, just so long as you kill them in one shot.
Performing a 2 for 1 bullet kill only adds 8% despite you killing two people in quick succession, not 16%.
A land-mine kill will score higher than a trip wire kill.
Also read somewhere that score depends on how many vital organs you hit with a single shot
, but I couldn't find any evidence to support this.
The main three factors seem to be Range
, Hit Location
and 'Number of Kills in One Shot`. Other than that, there doesn't seem to be an exact formula (even rough)