Additional Gun Mod Recipes [WIP]

Adds crafting recipes for gun mods like rifled and ported barrels, stocks, scopes, lasers, etc. WIP as all get and needs a ton of feedback on things like:

Recipe source books (current paradigm is main skill texts provide the basic fundamentals, weapon-specific books would probably include things like descriptions of parts and accessories from which you sort of mentally reverse-engineer them, hence the increased difficulty for those ones. Maybe a new book like an army field manual on equipment maintenance or an insurgent's guide to gunsmithing would be better.)
Difficulty levels (I might've highballed them compared to the difficulties for creating other similarly-complex items like revolver shotguns and finger laser CBMs)
Tools and resources required (Kevin mentioned some things like barrels and mechanisms would require a proper machine shop to fabricate, I built these based around the kind of stuff you find in gunsmith stores like welders and soldering irons, swage and die sets, hacksaws, etc so this might only work if we can get machine shop furniture and vehicle rigs set up and additional tool qualities like grinding, sanding, etc)
Pretty confident these are machine-shop territory too.  (That and I'm not fond of 'em as currently implemented: shouldn't just be able to slap on a pack in 6 seconds and instantly have a rechambered gun, with ammo converted in the bargain!)