Slick1998

If you have a crossbow, you can get sniper mode on any weapon, and here's
how: Simply quick-save your game, go to the crossbow and zoom in, and then
quick-load. When the game pops up again, you will be zoomed in on the
weapon you had when you saved, and can cycle through the others as normal.
To zoom out, just go back to the crossbow and press 2nd attack.
OR
Enable the console by
typing "-dev" or "-console" (without qoutes) as mentioned in one of the
above codes.
Start up the game, and enter a new game, or load a saved one. Now hit the ~
key to bring up the console. Type the following, where * is the key of your
choice. (Keep in mind, you must bind all three commands to different keys.
bind * "fov 20"
bind * "fov 50"
bind * "fov 90"
The first will bring you in to a very close zoom. Second is mid-range, and
third is the command to bring you back to the normal view. It works with
any weapon, though it will not change the mode of fire or increase/decrease
accuracy.