A replacement cupseal for a Phantom powertube with the brass cup support.
Why PTFE?
Above 90 degrees, it's likely your CO2 will go supercritical, making it penetrate elastomers extremely well, plucking free carbon from whatever material it's in. In a game? That looks like a swollen, gooey cupseal that gives erratic velocity and ruins your day. PTFE is both hard enough and inert enough to resist swelling and provide consistent performance. Way better than buna or cheaper urethanes (although the old clear CCI seals never gave me trouble).