Jahnsson came seventh in the Men’s 10 metre air pistol qualifications. In the final he scored a total of 669.2 points falling short of a medal.
The gold went to Jin Jongoh of South Korea with 677.4 points.
The London Olympics were declared open on Friday by Queen Elizabeth II. Finland’s team marched into the stadium led by swimmer Hanna-Maria Seppälä holding the Finnish flag.