To make a "found schematic" you actually need to have two separate things: the ingredients to make the new item, and the appropriate skills. If you look at the schematic display you will see that above the each picture of a required ingredient there is a small symbol and a number. The symbol indicates which skill (I think it is always the skill related to that ingredient) and the number the level of skill. Whether you can make the ingredient in question is not actually important - if you can get it some other way that's fine. You get approximately ten points of skill per level in that skill.
For the acid gun you mention you actually require level 6 in chemistry and level 4 in gun smithy, which happen to be sufficient to make the ingredients. For an Automaton you need Mechanics level 7 but only Smithy level 2 - you can't get the Elite Platemail ingredient any way but make it (that I'm aware of), but the skill to make it isn't actually required for the Automaton.
Lastly some found schematics only require a single skill. For instance the Flow Disruptor shield only requires level 6 electrical, and all the ingredients are available from shops.