The Tesla cannon is a pre-War, portable, directable energy weapon. It was designed to be used as a lightweight, compact anti-tank weapon. The design was named after Nikola Tesla, whose designs for the technology were seized by the U.S. government after his death. When used, the cannon projects a blast that will damage the enemy on impact. It will continue to gradually reduce the enemy's health due to its ability to electrify the target and its surroundings.
The Tesla cannon is a shoulder-mounted energy weapon that is powered by electron charge packs. This weapon has a high Strength requirement for its weight and the fact that it's a recoilless energy weapon. The sound of the Tesla cannon firing has also been changed from the one in Broken Steel. The Tesla cannon is considered a unique class of weapon and therefore cannot benefit from the Jury Rigging perk.
The Tesla cannon is rare, especially so if the Courier kills the Van Graffs, thereby eliminating one of the only specialty energy weapon vendors in the game.
After completing the Honest Heartsadd-on, the cannon will respawn in the Brotherhood safehouse footlocker.
Because the Tesla cannon uses 5 electron charge packs per shot, it's only possible to get 1 empty electron charge pack from firing the cannon, the same with all energy weapons that use more than 1 cell per shot.
Due to it previously firing only 1 shot per reload, it ejects single electron charge packs every shot despite only one being loaded. Also, the electron charge pack will not appear after one shot is fired, and the end of the cannon is retracted, but it still ejects another per shot.
The Tesla cannon was originally to be featured in the canceled Fallout: Brotherhood of Steel 2, bearing no resemblance to the weapon in Broken Steel. Described as a "high-powered mass-cannon throwing tessellated plasma forward in an orgy of death and destruction" in the Fallout: Brotherhood of Steel 2 design document, it was to be the so-called "uber-weapon" of player character Maxus. "Uber-weapons" were to be unique weapons for each player character which had to be assembled from 5 parts over the course of the game, or the player could have a NPC assemble one for them. One of the five parts for this weapon was a Tesla coil.
Nikola Tesla really did try to create a directed energy weapon but it would have been a particle beam weapon called a "Teleforce," something that would be immensely complicated to create.