Choong had looked like posing a challenge to the likes of Russia's Alexander Lesun and Ukraine's Pavlo Tymoschenko in his first appearance at an Olympic event, before struggling with the pistol in Brazil.
"Everything went really well except for the shooting," said Choong, according to BBC Sport.
"I was very happy with my first shoot and the last was the same but the middle I don't know what happened.
"There's been some positives. The girls coming back to finish top 10 and Jamie did well coming back. His swimming was close to his PB, which he hasn't done for four years."
Choong's ninth-place finish saw the 21-year-old improve on his finish of seventh at the European Championships.