WASHINGTON — One of the first votes new Democratic Sen. Martin Heinrich cast was against legislation from Sen. Dianne Feinstein to reinstate an assault weapons ban in the aftermath of the Sandy Hook school shooting.In the decade since, as mass shootings have touched almost every corner of the United States, the New Mexico senator, an avid hunter once endorsed by the NRA, has been considering what it would take to draft legislation that avoids banning guns that Americans use for legitimate purposes while still saving …