Travel to Nellis Air Force Base and gain access to the location by talking to the guard. Agree to go willingly with Raquel, who escorts the Courier to the eldest of the tribe, Mother Pearl. Pearl can be asked about the firing mechanism for the howitzer, but she is only willing to give it to a friend of the tribe. This leaves the option to either complete the Volare! quest in order to become liked by the Boomers, or to steal the firing mechanism from a gun cabinet inside the Nellis workshop.
Regardless of the chosen method, return to Fortification Hill after obtaining the firing mechanism. Search the northern cliffs for the howitzer, and interact with it in order to replace the firing mechanism. Return to Caesar's tent afterward, and inform Lucius that the howitzer has been repaired, which concludes this quest and awards Caesar's Legion fame as well as 100 XP.
Get a replacement firing mechanism for the Legion's howitzer from the Boomers.
Retrieve the firing mechanism from the Boomers' workshop.
Install the replacement firing mechanism into the Legion's howitzer.
Inform Lucius that the howitzer has been repaired.
Repairing the Howitzer for the Legion triggers additional scripted incoming fire and explosions during the Second Battle of Hoover Dam.