anonymous asks: What does Percy add too his tinker rolls other than his proficiency bonus?
Updated: It depends. From the D&D website: “Tool use is not tied to a single ability, since proficiency with a tool represents broader knowledge of its use. For example, the DM might ask you to make a Dexterity check to carve a fine detail with your woodcarver’s tools, or a Strength check to make something out of particularly hard wood.”
For fixing his guns in the middle of combat, we imagine Matt would require a dexterity check, under the idea that Percy needs to be quite nimble with his hands with a device with which he is already rather familiar. For crafting new, never-before-seen pieces of technology, we imagine an Int check is required to figure out how they would work in the first place (and @chaikovsk points out that characters who have offered to assist have needed to make Int checks, as well).