It isn't a federal law, the bump stock ban was a Trump presidential order. And an unconstitutional order at that.
The surprising part was that the conservative Supreme Court refused to hear the appeal on the bump stock ban. Since Trump has lost over 90% of the legal cases against his unconstitutional presidential orders, it was quite shocking the court didn't take up this one.....Obama was quite clear when he refused to enact a bump stock ban while he was president because it isn't in the President's purview to do so, legally such a ban is supposed to be enacted by Congress as a federal law and falls outside of presidential order powers. The Supreme Court should have heard the case and overturned Trump's order and referred the topic back to Congress.