This is a missable achievement.
Towards the end of the game, you'll decide whether to load a gun with real bullets or blanks. You need to load the gun with blanks.
Pass the gun to Hastings after loading it with blanks, and the achievement will unlock after a short conversation.
If you've deliberately loaded the gun with real bullets to commit a mistake towards the "Large Donkey" achievement, you'd need to start the game from scratch to unlock this achievement.