The military base west of the cinema has this plan. The plans are in the room where you fight the second grenadier. This room has Secret Files 6 and the communication panel to contact Serpo. This blueprint is a quest reward for completing Little Prince in the Jungle.
Locate the settlement near the broken overpass at the north end of the Jungle map (the overpass camp on your Fast Travel map) and search for an NPC named Claire. He'll arrange for you to find his son, who's in River Village. The map of the shotgun ammunition is in the police station; it's in a file cabinet in one of the rooms. If I remember correctly, you found it in the burning building outside Henderson when you were following the bloody tracks after the helicopter incident.
However, it creates 15 rifle bullets, more if Purna has part of her skill tree full, which makes rifles more viable to use constantly, especially if a new campaign is started after pre-completing it with a character. After finding a guy who always sells Metal Scrap, I decided to look for the ammo mods so I could create my own ammunition and annihilate zombies. This makes crafting ammo using the blueprint quite useless, as you can easily travel back to the group's safe house and replenish the free ammo there. Unlike other modified planes, when used on a workbench, it does not create a new weapon, but rather creates ammunition for the rifle.
In addition, rifles are the second most difficult firearm to obtain, with very few enemies carrying it and only a few merchants selling it, so the probability of having a rifle as a main weapon is low. Unlike the rarity of the weapon, several merchants sell rifle ammunition themselves, including Wes Tweddle in the Bloodbath arena, which can be easily accessed from any Fast Travel map and a good alternative to save bleach. I have the others related to the shotgun (Pride, Shock, Detox, Hooligan, etc.) but I have no ammunition other than Rifle and Pistol. It is a decent mod for the rifle and can cause incredible damage when combined with the higher-powered rifles.