Gunsmithing, Specific Information:
Wikipedia - A good overview of gunsmithing such as jobs and job descriptions that are available to all gunsmiths.
Practical Machinist Forum - All budding machinist and soon to be gunsmiths will love the insight on this site. Gunsmith machining questions answered here, very informative.
Amazon.com - Gunsmithing books with extremely detailed information that any professional gunsmith will appreciate.
The NRA Gunsmithing Guide - Updated
Rifles - Gunsmithing guide covers; millett scope mounts, rifle scopes, rifle scope mounting, rifle barrel cryogenics, and rifle barrel free boring.
Shotguns - Gunsmithing guide covers; shotgun bore burnishing, shotgun barrel forcing cones, shotgun bore back boring, and shotgun stock fitting.