New York Mayor Michael Bloomberg is losing part of his arsenal of local leaders as more and more long-time members of his gun-control group Mayors Against Illegal Guns say they aren’t happy with the coalition’s trajectory and want out. While the group apparently is growing in membership overall amid an effort to assume a larger profile in the national gun debate, it's turning some members off. In the past five months, 50 members of the group have quit. Many say they did so because the organization abandoned its mission statement of going after illegal guns, and instead used its political...