PTFE Tape

Known to science nerds as Polytetrafluoroethylene, PTFE tape is used to create an airtight seal around threaded parts, specifically the threaded end of a Worker barrel.
Whip a couple of rounds of this non-stick tape around a thread and it will create a tight seal, stopping any air squeezing past.
PTFE tape can also be used in lieu of a properly sized o-ring if you're trying to beef up the air seal in your plunger tube.
An essential addition to any nerfer's tool kit.