In this page you can find information about locations of important plans (schematics) that are necessary to craft shotgun and pistol ammo by using fabricators. Acquiring those plans is recommended if you play on one of the higher difficulties and/or your gameplay style is about ignoring human abilities (which is required for one of the achievements/trophies). When you play like that you can easily deplete all of your ammo.
Warning - Information about locations of other schematics can be found in Plans chapter.
Shotgun Shells Fabrication Plan can be found in the same location in which you find the shotgun during the first few hours of the game. This plan is found in Security office which is located on the level 1 of Talos I Lobby (you visit this location for the first time when you use the passage in Neuromod Division).
The Security office can be accessed by opening the door (access card is in IT security office on the level 2 of the lobby) or by walking on the yellow pipe above the door and using a small vent shaft. When you reach the Security office you must find the safe, presented in the picture, and open it by using the following password: 0526.
Silenced Pistol Ammo Fabrication Plan (9mm) can also be acquired during your first visit in Talos I Lobby. In this case you must go to Teleconferencing Center located on the level 3. The access key can be found in Yu's office. The plan can be found by examining Elias Black's corpse, presented in the picture, but before you examine it you must defeat the nearby Mimics.
If you missed your chances for finding those projects in the lobby and you don't want to go back only to get them you can still get BOTH ammo plans when you are in Psychotronics. You must get to the armory located on the level 2 next to Material Extraction Lab. The door to the armory are closed but you can open them by using this access code: 8714 (you can get it from Aaron Ingram who is imprisoned in a cage).