The 32-year-old former world speed shooting champion missed only one bird in the first 25, but slumped to 20 the next time before recovering with a 23 for his first-day output which put him seven points away from co-leaders Giovanni Pellielo of Italy and Russian Alexei Alipov, who had 74 each.
American Bret Erickson led five others with 73s. They are Finland’s Petri Nummela, France’s Ahmed Almaktoum, Peru’s Francisco Boza and Lance Bade, also of the United States.
Australian Michael Diamond, back-to-back Olympic gold medalist (Atlanta and Sydney) was tied for ninth spot at 72.
Dionisio competes anew Sunday morning in the final 50 targets of the qualification round.
The top six shooters after 125 targets will vie for the medals in 25 more targets.
Meanwhile, reigning Southeast Asian Games 1500-meter freestyle champion Miguel Mendoza checked in 35th place in the 400-meter freestyle after clocking 4 minutes 1.99 seconds in the 400 freestyle.
"I’m quite happy with my time," said Mendoza, who actually used the event to warm up for his 1,500 competition.
Mendoza’s best time in the 400 is 3:59.07. "But I have not been swimming the event anymore, except now. In Vietnam, I clocked 4:03 during the Southeast Asian Games," he explained.
Another Filipino, Miguel Molina, timed 4:33.25 in the 400-meter individual medley to finish last in his heat.