"The Chinese guy won the gold last time. I promised myself that I would not let it happen this time," Jin said.
That guy, defending champion Pang Wei, ended up fourth. Pang went into the final lying second, but dropped out of the medal spots when he failed to bulls-eye with his seventh and eighth shots and couldn't make them up.
Jin, who led by two after qualifying, appeared set to cruise home as he increased his lead to four halfway through the 10-shot final.
But he missed the innermost circle four straight times, cutting his lead to less than two. He stood at the far left of the range so every one of his fellow right-handers was watching his back.
"I was very nervous," he said. "I did my best to overcome my anxiety."
With his last shot, Jin pulled off the best score of the final, a near-perfect 10.8, to leave no doubt. He raised his arms in silent celebration as others were yet to finish.
Jin totalled 688.2 for his fourth Olympic medal and second gold. He will defend the 50-meter title next Sunday at the Royal Artillery Barracks.
Italian policeman Luca Tesconi, who has family with him but told them not to watch, rose from fifth for the silver medal in his first games.
"It's the best day of my life," Tesconi said. "I closed into a bubble of concentration and relived the experience of training at home."
In that bubble he didn't realize he'd finished.
"I thought I had another shot, then I looked at the screen and I saw my name, Luca Tesconi, silver."
Andrija Zlatic of Serbia was steady for the bronze, scoring 685.2, just six-tenths of a point from matching Tesconi. It may take until Sunday, Zlatic said, before the medal sinks in.
"It depends if I can fall asleep," he said.
Oleh Omelchuk of Ukraine was the only shooter to hit 10s with his first seven shots but fell out of the medals when he made an 8.5. European champion Pablo Carrera of Spain was sixth.
The anticipated showdown between top-ranked Tan Zongliang of China and world champion Tomoyuki Matsuda of Japan never eventuated as they failed to reach the final. Tan was 12th and Matsuda 13th, both two points shy of the top eight. They still have medal chances in the 50.