I've had a batch manufactured in 90 Shore Polyurethane, no CO2 absorbtion, no distortion on decompression, long life etc.
See here -
http://umarexboysclubforum.myfinefor...1253.php#31253
Airgun Repairs, Bespoke Airgun Smithing and Precision Engineering Services
http://www.magic9designltd.com