We know that laws preventing the sales of assault weapons work — the federal ban in place from 1994 to 2004 was proved to reduce shooting massacres in the United States. But if Congress is unwilling to act on the issue, it falls to private industry to do so. Under federal law, you must be 21 years old or older to purchase a handgun. But to buy a long gun, including a semi-automatic rifle, the purchaser only needs to be 18 years old.