BANGKOK, Thailand — Few weapons marry convenience and raw power like the M-79 grenade launcher. Known to Vietnam War-era G.I.’s as the “Blooper,” it can lay waste to cement walls or crowds only to discretely vanish into an attacker’s backpack. Throughout the past two years in Bangkok, this 28-inch menace has done just that. Just as Anthrax letters evoke post-9/11 paranoia in America, the Blooper has come symbolize politically motivated terror in the Thai capital.