In addition to causing horror and outrage, Friday's shooting deaths by Adam Lanza of 27 people, including 20 school children, in Sandy Hook, Conn., are already prompting calls for new gun laws. Supporters of gun control demand restrictions on ammunition magazines and bans on so-called assault weapons—semiautomatic versions of military firearms. Gun-rights advocates see another mass shooting in a "gun-free zone" and argue for expanding the places where individuals with valid permits may carry their weapons.

Neither...