However, many shootings are not the result of such determination, but are the result of impulse.
Absolutely, a determined person can 3D print a 30 round magazine. People line up at machine shops to make their own AR-15 lower receivers. These sorts of resourceful, determined, goal-oriented people, are not in general responsible for a lot of shooting deaths.
It is truly surprising to me how many gun dealers are hanging by such a shoestring that the only thing that keeps them profitable is the sale of 30 round magazines. But you apparently know a lot more about the market than I do.