One of the most promising rifle shooters from the state in the recent times, Koshy claimed the victory in the home state ahead of Haryana's Meena Kumari and West Bengal's Kuheli Ganguli.
She bagged the gold with 618.5 points, while Meena Kumari picked the silver with 617.8 points and Ganguli settled for a bronze with 614 points.
Hailing from Thodupuzha in Idukki district, Koshy said she never expected a medal but wished for one in the Games as the event is being held in her home state.
The silver medallist in Asian Shooting Championship, Koshy said the victory here is special as she beat many bigwigs including Olympians and world champions in the group.
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Koshy said she is now hoping for a medal in the rifle 3 position.
National shooting coach Sunny Thomas said the medal was expected in the category as Koshy had shown excellent performances in many recent championships.
A participant in Glasgow Commonwealth Games, Koshy also represented the country in World Cups in 2013, 2014.