Howitzer

The 75mm howitzer is a pre-War weapons system that was used by the United States Armed Forces. A large number of these weapons was stored in various arms depots across the country, useless to anyone without proper training.

Post-War use
Algernon in New Reno has been provided with a howitzer by Eldridge for repairs (although what use could he have from such ordnance in his basement is unknown).

The Boomers acquired several of these guns from Area 2 to protect Nellis Air Force Base from enemies.

Caesar's Legion has acquired a howitzer for the Second Battle of Hoover Dam, but without a Howitzer Firing Mechanism, they've been unable to coax it to working order.

Appearances
A working howitzer and a slew of unusable ones appear in Fallout 2. Working howitzers appear in Fallout: New Vegas.