AUTODYN: pressurized gas inside a vessel

Hi, 

is there any way for EXPLICIT representation of pressurized gas (e.g., air) containing inside a vessel using ANSYS AUTODYN or Explicit Dynamics? If gas is modeled using SPH in AUTODYN (or even Lagrange elements, which doesn't matter), how to make it actually press on vessel's wall and create stresses there? I tried to use pressure shift in ideal gas EOS - it doesn't give any reasonable result. Explicit representation of gaseous media is needed to conduct shock waves through it, as, obviously, amplitude of those waves will depend on gas density determined by pressure. Idea with artificial temperature expansion doesn't seem to work either.