but you still need to buy the actual firing mechanism parts of real guns in order to manufacture

Nah thats the parts the jigs make, as well as a couple other key pieces that require higher than usual precision. At least, if you want to actually make a gun that can reasonably hit a target.

And you can also make those same jigs and fixtures out of wood or any other raw material.

Not by hand with the precision needed, not for the parts in question. Unless you want to risk a misfire and losing a finger.