lredmond@lowellsun.com BOSTON -- As a Billerica gun case heads to the state's highest court next week, the Brady Center to Prevent Gun Violence, the International Brotherhood of Police Officers, Police Chiefs and other high-profile groups are urging the Supreme Judicial Court to reject the constitutional challenge to the state's safe firearm-storage law. The groups are joining with Middlesex District Attorney Gerard Leone, who is challenging a Lowell District Court judge's ruling that dismissed a gun-storage case against Richard Runyan, a Billerica man who kept an unlocked semiautomatic hunting rifle under his bed where it could be accessed by his teenage...