That means, that he fired the gun empty, and then stopped to reload so that he could shoot each of them again.
11 seconds sound clip from the The Shawshank Redemption movie soundboard.
You can hear this line at 00:05:58 in the Blu-ray version of the movie.
Quote context
[...]
- Now, while you think about that. Think about this.
- A revolver holds six bullets. Not eight.
- No. This was revenge of a much more brutal and cold-blooded nature.
- Consider this. Four bullets per victim. Not six shots fired, but eight.
- An extra bullet per lover. Right in the head.
- You strike me as a particularly icy and remorseless man. It chills my blood just to look at you.
[...]