Whether they exist in an area the player can get to is of course, another question entirely. This leads me to believe that the Shotgun Ammo Blueprints are located in the interior military base level.
If you use something to grep the files, like Windows Grep, and use the search string 'Craftplan_Shotgunammo', you'll find there is exactly one file where it's found - data/maps/bs_military_quarters/bs_military_quarters.exp
If you unzip the entire data1.pak, which contains all the game level information, you'll get an assortment of files (some compiled, some not) that tell you what's in what level. Lo and behold, you'll find it - so it's 'there', the question is, where in the game? You can check this yourself by unzipping data0.pak, then opening data/collectables.scr in notepad or something similar, and finally doing a search for 'Craftplan_Shotgunammo'.
So it turns out, the Shotgun Ammo Mod is in the game - or at least, in the code.
