Borderlands natively supports widescreen and ultra-wide resolutions through its options menu. Support is normally Vert- in all widescreen resolutions as is typical with Unreal Engine 3 games, however there is a quick solution for Hor+ widescreen gameplay in all resolutions.
Step One: Download the hack from files available on PC Gaming Wiki
Step Two: Run the hack and set your FOV in the middle-bottom section. You must set the FOV change to a key from the list below that, such as the forward movement key or sprinting key. When in-game the FOV will "snap" to the Hor+ value at the press of this key.
Step Three: Hit apply and close the hack, then run Borderlands as normal and press your set FOV changing key.
Note that using a vehicle in the game resets the FOV as may some other events like a cinemas playing. With the FOV set to a movement or sprint key you can quickly reset it to the Hor+ value by moving in the game after the car or cinema section is complete.
Borderlands has what some might call a critical flaw in widescreen resolutions using the above widescreen method. When sprinting Borderlands alters the FOV for a kind of reverse zoom effect to make you feel the sprinting effect. When hacking the FOV for Hor+ widescreen the increase in FOV when sprinting can be extreme depending on your resolution. I personally find it to be perfectly acceptable in 16:9 resolutions but the wider your aspect ratio and thus the higher your FOV the more the sprint effect will feel "fisheyed" and disorienting.
Refer to screenshots of normal Vert- sprinting followed by Hor+ sprinting, first in 16:9 and then in an ultra-wide resolution of 1920x768.
=====
Some people may experience a dramatic loss of framerate when moving the mouse in game. This is due to high resolution gaming mice being used on Nvidia systems. Mice of up to 1000DPI have been tested ok, but 2000DPI mice tested all prove to have this stuttering low-framerate problem when using Nvidia GPUs. Even dropping the mouse DPI using software does not seem to work, only mice that can drop the DPI in the hardware will work if stuttering is caused at max DPI.
