http://tech.airsoftcanada.com/TM/TM%...26diagram2.jpg
From the diagram, it looks like the O-ring is right at the spot you identified as being the source of the leak, so if I were you, I would just just unscrew the bottom, pull it out, flip the ring, clean any dust and grim off the metal (NOT rubber) pieces with rubbing alcohol + qtip, put back together tightly and put the screw back in firmly (not TOO hard, though).