This took a while to figure out. There are numerous ways of handling weapons and armor in Fate Core. Balancing out the costs and how they work as extras is a pretty big deal, and is something there aren't a huge number of guidelines for. Fortunately there are a lot of ideas out there. The one that I went with was from this post on EldritchFire Press which synced up with some other suggestions I'd seen. I think the end result has a good amount of abstraction without making the weapons and armor entirely too fiddly (and, having looked through some GURPS and Hero books lately, I definitely know fiddly).
You can grab the draft document here. It still needs some details regarding "enchanted" artifacts and whatnot, which I'll be filling in later.