https://www.80percentarms.com/products/0-billet-ar-15-lower-receiver/
You illustrate the point perfectly with this product as an example: Where does the limit of vagueness end? By Shapiro’s own words, a firearm is to be defined by its physical shape. There is nothing else substantial — or even logical or measurable — in his brief. My hope is his office becomes inundated with examples of “homemade firearms”, because by his words, damn-near everything capable of withstand a little heat and pressure could be considered a firearm.