This photo?
Gen. Nguyen Ngoc Loan executes NLF/VC Captain Bay Lop. The VC was captured after he and his men had killed one of General Loan's senior officers and his entire family during the Tet Offensive in 1968.
Sometimes a .38 will have to do when you don't have anything...slower.
The photographer later apologized to Gen. Loan for the trouble this picture caused him and his family.