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i have checked the digital map time and time again and I have not been able to locate the shotgun ammo bueprint.
My guess is that it is some side quest or it was not put in due to a mess up or something of the sort.
I am afraid it may have been left out by mistake.
In DI1 they accidentally left some blueprints out when it first came out and they patched them in later, so I think you're probably right because I just finished the game and I'm pretty sure I scoured every inch and did every side quest