OTTAWA -- The Liberal government is expected to announce this week an ambitious program to buy back legally owned firearms that it last year deemed to be assault rifles. Legislation implementing the buy-back program would make good on a commitment the government made in May in response to the mass shooting in Portapique, N.S., that left 22 people and the gunman dead. After the shooting, the government reclassified as “prohibited” about 1,500 different firearm models and their variants, including the AR-15 rifle that was used in many mass killings in the U.S., and the Ruger Mini 14, a hunting rifle used in the Ecole Polytechnique massacre in Montreal in 1989.