tobnpr, you are spot on. Any part that must be made, even a spring or a pin, would fall under their definition of manufacturing a gun part. Under the definition, one would only be allowed to replace those parts with pre-made parts from a manufacturer, thus they are staying away from armorers. Of course they will stay away from them, because local law enforcement agencies hire them, and they would have to purchase a license if not for that. I'm sure they would love to stretch the definition even farther if they could, if not for that.
Since they purposely single out stock making, because machining is required, that will also single out bedding, because inletting of the wood is required, and it also makes the gun more accurate. To get out of the license requirement, one is reduced from a gunsmith to an armorer.
What is needed, is for congress to add the definition of a manufacturer to the law, via a bill. That definition would need to state that the word, manufacturer, within this law, is defined as a business who builds complete, and operating firearm(s) from raw materials, and that any other work, machining, or repair operation of firearms is to be considered non-manufacturing. That would equal the BATFE definition.