If it is part 31, then it is your main seal like you've stated. This piece resides inside the gas nozzle, inside the bolt. You will have to take your bolt apart, take off the cylinder, the return springs, and will find a pillar type object with a screw in the center. You will have to fit this rubber piece (seal) back around that housing, probably by unscrewing that screw and placing the seal back in place. If it came off, there's probably a tear in the seal so I suggest buying another one, and fitting it onto the "pillar". It goes around the outside of the top, and provides a seal around the gas cylinder to ensure no gas is expelled out the other side of the nozzle. Of course, if your gun has a little gas spew out, it's generally not an issue. Since green gas has silicon in it, as you shoot the gun, the green gas will lubricate this seal and ensure it provides a nice, tight seal around the gas nozzle.
Edited by Outlaw1995, 12 September 2012 - 01:31 PM.