Massachusetts lawmakers are weighing tougher gun control measures, from requiring gun owners to buy liability insurance to setting tighter standards for firearms licenses.  State Rep. David Linsky, a Natick Democrat, said he plans to file by mid-January what he called a comprehensive bill to end gun violence.  Linksy spoke with reporters Thursday moments after meeting with more than 100 lawmakers and legislative staffers of both parties.  Linksy said other ideas being debated included allowing mental health screenings and tightening rules for storage of firearms.  Gov. Deval Patrick has already proposed limiting gun purchases to one per month and toughening laws aimed at ``straw purchasers'' of guns.   Gun rights advocates say lawmakers are focusing too much attention on law-abiding gun owners and not enough on criminals.  Massachusetts already has some of the nation's toughest gun laws. (Associated Press)