Howitzer

The howitzer is a world object in Fallout 2, Fallout: New Vegas and appears as a unnamed artillery piece in Fallout 3. The model of the howitzer (NellisArtillery) is used in other artillery pieces.

Characteristics
The howitzer and other artillery are pre-War weapons that were used by the United States Armed Forces, some are mobile, some are stationary but all use the same basic principles and gunpowder as a propellant. They provide covering fire for ground forces, anti-air support, can soften up fortified targets or fill a role in testing new varieties of shells (most notably Saturnite).

A large number of these weapons were stored or fielded in various parts of the country, useless to anyone without the foresight to see an extremely valuable piece of weaponry.

Fallout 2

 * A howitzer can be used to gain access to the Sierra Army Depot, providing a wealth of resources.
 * The savant weapon smith Algernon in New Reno has been provided with a howitzer by Eldridge, the owner of the New Reno Arms, for maintenance. The weapon is to be added to his collection and stockpile of arms.

Fallout 3

 * The Talon Company possesses an unknown number of artillery pieces. They are never strictly identified as howitzers, yet given the level of pre-War technology at their disposal and the rarity of other forms of artillery, it is reasonable to assume the weapons are howitzers. Artillery strikes from the weapons can be called in on Seward Square and the super mutant behemoth at Takoma Park. Both include some form of operating instructions, which is a note near the signal switches.
 * The Enclave possess artillery in some form as well, as evidenced by the Sector artillery notes located at Adams Air Force Base. However, these weapons are only mentioned, as they do not have a means of being fired. The Enclave will instead fire from atop the Mobile Base Crawler using their Big Guns.

Fallout: New Vegas

 * The Boomers acquired several of these guns from Area 2. They are used to protect Nellis Air Force Base from strangers near and far, as well as preventing anyone from effectively spying on the base. Caesar mentions that some of his scouts were shelled at extreme distances. The Boomer guns were also used in unspoken negotiations with the New California Republic, who had attempted to shut off the base's water supply. In response, the Boomers shelled the water lines to the NCR Sharecropper Farms, prompting the NCR to turn the water back on at the base.
 * Dale Barton salvaged a howitzer from a military base in Yuma, Arizona and then sold it to Caesar's Legion. The Legion hopes to use it during the Second Battle of Hoover Dam for suppressing 1st Recon and Veteran Ranger snipers, but without a Howitzer firing mechanism, they are unable to coax it to working order.
 * There is also a howitzer in the wreckage of Hopeville Missile Base West, the cannon can be found near the Marked Men Camp. The howitzer  is part of the debris that makes up the barricade next to the  mountain wall.

Anti-aircraft gun
An anti-aircraft gun is positioned at the top of Hoover Dam, the dam would have been especially vulnerable from the air during the Great War, however the air strikes never came and full scale nuclear war broke out instead. Its likely the New California Republic keep the gun in working order to prevent a surprise attack by anyone who manages to salvage pre-War technology or those who have post-War technology, such as the Enclave. Despite this fact, it is not used by them during the Second Battle of Hoover Dam to stop the Boomers or Enclave Remnants from utilizing their aircraft as it is destroyed by the Legion early in the battle.[1]. The gun can also be rigged to explode or reprogrammed during the Caesar's Legion quest Arizona Killer to assassinate the NCR president, Aaron Kimball.

Fixed artillery
Several fixed guns were positioned in Alaska to help safeguard the Alaskan Pipeline, however when the People's Liberation Army invaded the guns were seized and turned on the United States Armed Forces. During Operation: Anchorage the guns were finally silenced. How exactly control or destruction of the guns was carried out is unknown, as the only surviving record was a simulation which may or may not be entirely accurate in its portrayal of events. The guns make another appearance in Big MT, at the X-7a "Left Field" artillery launch. The guns at Big MT fire test rounds comprised of Saturnite (a space age material), one of these shells can be used in a test firing targeted at the X-7b "Boom Town" target zone. The resulting crater reveals a hidden cache that the player can loot.