Even with the precautions make sure not to loose the nub this piece is just a long tiny cylindrical piece made out of rubber.

This piece rests on the hop-up arm and is most likely still in the hop-up arm however loosing this piece is the most common mistake done by people disassembling their hop-up for the first time.

This is now a good time remove the bucking if you desire.

When you go to add the bucking to the barrel make sure the long protruding section of the inside of the bucking with the slot of the outside of the inner barrel.