Legislation allowing veterans classified as “mentally defective” to buy firearms is drawing sharp criticism from gun-control advocates, but proponents argue that it is not as risky as it may sound. Eighteen senators — including Sens. Pat Roberts and Sam Brownback of Kansas — have endorsed the measure. It would require a legal authority to find a veteran is a danger to himself or others before his name is put in the national criminal background check system and he is prohibited from purchasing a gun.