Okay, there's this one locale....

http://fallout.wikia.com/wiki/X-7a_%22Left_Field%22_Artillery_Launch

...which we see in the promo trailer actively firing its monstrous battleship-sized guns. So do we actually have an opportunity to fire them, or should we http://tvtropes.org/pmwiki/pmwiki.php/Main/NeverTrustATrailer?

Yes both pieces can be fired.

Should have read the wikia page that you linked:

"On the platform in between the cannons, there is a console where you can activate a "Test-Fire" sequence which shoots one of the cannons, revealing a crater in the south end of the X-7b "Boom Town" Target Zone with an Atomic-valence tri-radii-oscillator inside a locker"

:P