At the end when you are allowed to choose a location to fire the weapon you are given five choices. The Citadel, Project Purity, Megaton, Rivet City, and Adams Base.
It will not let you fire on either Project Purity, Megaton, or Rivet City, but you can fire on the Citadel or Adams Base. I have confirmed that firing on the Citadel also gives the achievement. But be warned, choosing the Citadel gives evil karma.
I'd honestly save the game at the up-link terminal so you can see both endings. Choosing the Citadel is quite funny.