WASHINGTON - Armed airline pilots may be at risk of discharging their weapons on planes because of cumbersome rules imposed by Congress and the Transportation Security Administration, a group representing the pilots said Tuesday. A 2002 law prohibits pilots from carrying their guns outside the cockpit without a trigger lock, and as a result they must frequently affix or remove trigger locks during flights. That law and the TSA policy that goes with it may be responsible for a March 22 incident in which a pilot accidentally discharged his weapon on U.S. Airways Flight 1536 from Denver to Charlotte, N.C.,...