Weapon Crafting Guide is supposed (according to its description) to affect many more buildings than it does.
Currently, on my island it's adding bonus only to sword makers (from bronze to platinum swords).
It's not affecting any other weapons besides swords: arquebus, cannons, crossbows, battle horses, horses (stable or epic farmyard).
I don't know about bows and longbows (as I don't have those buildings) nor mortars (not starting my foundry for testing purposes).
I'm sure it was working on arquebus in the past (as Weapon Craft + Machine Oil was the only way I could fulfill a certain achievement).