Mel Gibson JACKED -- Check Out My Lethal Weapons

Mel Gibson is only hitting one thing these days ... the gym ... just check out the diesel arms he unveiled in L.A. on Friday.

Makes sense. 57-year-old Gibson is set to play the villain in next year's action flick "Expendables 3" alongside equally yoked middle-aged actors Sylvester Stallone, Arnold Schwarzenegger, Jason Statham, and Dolph Lundgren ... so bulking up is kind of expected.