Zac Efron -- It's Gut Check Time

Zac Efron proved he's got game ... by flaunting his jacked-up rock hard chest on the set of his new flick.

With his shirt casually unbuttoned all the way, the toned 25-year-old stud played some basketball -- and showed off his ridiculously sculpted pecs and abs -- while filming "Townies" in L.A. on Tuesday.

Last month, Zac injured his right hand on set ... but everything appears to have healed quite beautifully.

Zac may be the one with the ball, but it's everyone else who has to worry about dribbling.