Original source
Does anyone know if the book/story this was based on can be purchased? Or was it a play or something?
shareUnder the main opening title, the source is credited as "Based on the short story 'RODOLPHE et le REVOLVER' by Noel Calef"; who is also credited for his story in this site's main page for the film.
Unfortunately the Wikipedia reference (in German) for this Bulgarian author is sparse and not helpful in this case. You could try Amazon.com.
That wasn't very sporting, using real bullets.