Ideal Gasket Material?

[Boomerang]

What is the best material for a barrel sealing gasket? Looking to play in the 100-200 psi range. Do I want something really soft like silicon? Buna-n, like Orings? Perhaps a target durometer?
Maybe I actually want 2 layers? A thin, soft material (perhaps silicon), over a thicker layer of something like UHMW polyethelene?

I don't mind spending a little money at McMaster or wherever to get the right stuff. Target of this platform is efficiency, so no problem spending a few more bucks just to be a little better.

[jackssmirkingrevenge]

Truck tire inner tube is an excellent source.