Alfred C. Johnson, 56, a senior firearms examiner with the Treasury Department's Bureau of Alcohol, Tobacco & Firearms, died April 19 at Shady Grove Adventist Hospital after a heart attack. He lived in Gaithersburg.

Mr. Johnson, who moved here and joined ATF in 1975, worked at the bureau's National Laboratory in Rockville. An expert on ballistics, he had examined evidence dealing with the assassinations of President Kennedy and the husband of Philippine President Corazon Aquino. He also was the recipient of an award from Arkansas for his work with a joint state-federal task force in combating a neo-Nazi organization.

Before joining ATF, Mr. Johnson spent 20 years with the New York Police Department, retiring in 1975 as a detective in its firearms identification section. He was the designer of the stainless steel water table system used in bullet-recovery and comparison operations by the NYPD. He also helped develop several optical systems that are now widely used in ballistics.

He was a native of Brooklyn and had attended the City University of New York's John Jay College of Criminal Justice. He served in the Army from 1949 to 1950 and again from 1952 to 1954.

Survivors include his wife, Harriet, of Gaithersburg; two children, Michael Johnson of Gaithersburg and Catherine Marano of New York City, and four grandchildren.