From: Officers in Flight Suits, The Story of American Air Force Fighter Pilots in the Korean War
"Far and away the greatest danger to the fighter-bomber were anti-aircraft (AA) weapons. The Communist AA effort was concentrated to cover the areas of the Chongchon River that the MiGs did not patrol. Bridges were especially well defended, but even along standard stretches of track, the Communist forces deployed anti-aircraft emplacements every four miles. However the weapons that pilots feared most were not these big guns, but small caliber 37-millimeter automatic weapons operated by regular Chinese and North Korean troops. Small caliber anti-aircraft weapons presented such a dangerous threat for the fighter bomber because they were indiscriminate and easily hidden in so-called 'flak traps.' Regional militia and repair troops would guard important interdiction routes by creating large barriers of small arms fire. These troops also strung wire cables between hills to thwart low-level attack, and created elaborate ambushes using tanks as bait to lure U.N. aircraft into carefully configured 'kill zones' of automatic weapons."
"Superior flying could not warn you of hidden flak, nor could it necessarily save you in a typical bomb run. All a pilot could do was minimize his exposure to fire by employing a steep angle of attack in a bomb run; however, exposure was still unavoidable. Such daily, involuntary risk taking made fighter-bombers much more fatalistic than their fighter-interceptor brethren, living a life of uncertainty and praying every day that their luck would hold out."