...in a firearms discussion forum. It is apparently not a very common term at all.I think that definition can apply to more than a firearm though. A car can be bone stock; actually anything you can modify could be considered bone stock, as long as it is straight from the maker.
Here's a definition from the entry bone-stock in the glossary at ClassicCars.com:
I think the use of bone here is an application of the adverb bone defined as follows in the Merriam-Webster Online Dictionary:Bone-stock
An original, unmodified car.
EXTREMELY, VERY <bone tired>; also : TOTALLY