"Congress may regulate trade across state boundaries"
This is exactly correct. Unfortunately, this has been reinterpreted and ridiculously broadened to mean any trade that would decrease or otherwise have an impact on trade between states. Anything that is made and sold in my state that otherwise could have involved a trade between states, thus reducing or otherwise affecting trade across state borders, then comes under federal regulation.
It gets worse. Home made items for home use also become subject to federal regulation. Under this definition me making a book case or shaping my own axe handle or growing my own food, let alone manufacturing my own class 3 weaponry, becomes regulateable by the federal government because it has an affect on the trade between states.