That's a good question, Hal. I know that seals in hydraulic cylinders that are shaped with a lip, similar to the bottom of some shotcup's rims can contain thousands of psi in liquid. I have kinda always assumed that they do a far superior job of sealing off the gases than the old card/fiber wads did. But, do they completely contain it, 100 percent? IDK.

May God bless America and those who defend her.