PTFE has a small band of devoted devotees. It is interesting to note that manufacturers have produced airguns with the following sealing materials:

Leather
'O' rings
Polyurethane parachute washers
Steel piston rings

None of them have used PTFE. It was experimented with by the tuning houses of the late 1970s and early 1980s and discarded. Various individuals have used it and some say it works great, but its ability to deform and its lack of ability in terms of sealing well when anything isn't 100% round means it will never be a popular choice.

T.R.Robb sells PTFE washers to the true believers and that's probably where it begins and ends in 2017.

I used it once and it was shit.