Mr. Biden correctly points out the frequent use of military-style assault rifles and high-capacity ammunition magazines in mass shootings and the degree to which they multiply the carnage. In Dayton, he notes, the shooter was killed by police less than a minute after he opened fire, yet that was still enough time for him to kill nine people and injure dozens more. He wants to write the new law more carefully than the old one to make sure gun manufacturers can’t skirt it by making minor modifications to their products, and he wants to institute a buyback program for the millions of assault weapons currently in circulation. (He has not gone so far as to suggest that buybacks be mandatory, as some, including former presidential candidate Rep. Eric Swalwell, have done.) Like all the other Democratic candidates (and sometimes even President Donald Trump), Mr. Biden would expand background checks to cover private sales of weapons, not just those by federally licensed gun dealers. He also talks quite a bit about encouraging the adoption of “smart guns,” which are designed to prevent their use by anyone other than their registered owner.