The artillery piece schematics is a world object in Fallout 4.
Finding those schematics allows the Sole Survivor to build artillery pieces at controlled settlements.
Once the schematics are picked up they will disappear from the world and will not appear in the Pip-Boy inventory.
The artillery schematics are found in the armory of the Castle armory during the quest Old Guns.
Behind the scenes
For additional real world information about this topic, check out the Independent Fallout Wiki's Real World article.