U.S. Representative Edward J. Derwinski (R-Ill.) suffered minor shotgun pellet wounds and his 14-year-old son, Michael, sustained superficial cuts and burns yesterday morning in a freak accident at a Bull Run Park trap and skeet shooting range in Fairfax County.

The two were rushed by ambulance from the park at 7700 Bull Run Dr. near Centreville to Commonwealth Doctors Hospital in Fairfax City. Derwinski, 55, was transferred to Alexandria Hospital for surgery to his injured right hand later in the day, said Laurestine Gravely, nursing supervisor at Alexandria Hospital. The congressman was listed in good condition yesterday. The younger Derwinski was treated at Commonwealth and released, said a spokeswoman for the hospital.

Derwinski, his wife Patricia and an aide were watching the young Derwinski practice at one of the park's six skeet shooting facilities when the accident occurred about 11:40 a.m., police said. The youth was using two 12-gauge shotguns and hand-loaded ammunition, the range manager said.

While attempting to aim his 49th shot, Michael's gun misfired, spewing burning gunpowder into his right arm and face, police said. The misfired shot apparently startled Patricia Derwinski, police said, and caused her to drop a gun she was holding for her son. When the gun, whose safety lever had been removed, struck the ground, it discharged pellets that struck the congressman in the abdomen, knee and right hand. He had been sitting on a bench 20 feet away.

Derwinski, who has represented Illinois' 5th District in Chicago since 1958, is the ranking Republican on the Post Office and Civil Service Committee. The congressman told park officials his family frequently practices at the shooting range, which is operated by the Northern Virginia Park Authority.

The Derwinskis, who live in the 4100 block of North River Street in Arlington, could not be reached for comment yesterday.