What's the maximum pressure inside a bombardier beetle?
According to this study, model data shows a maximum pressure of 110 kPa, that's 16 psi and only 1.086 atm:




  The three-phase process involved in the beetles explosive secretory
  discharge (ESD) process. The inlet size is shown as a proportion of
  the inlet radius. During the first phase of refill and heating (blue),
  only the inlet valve is open. During the second phase (red), the
  chamber is closed. Finally, the third phase (marked by an arrow) is
  the exhaust phase, when only the outlet valve is open. This phase is
  significantly shorter than the first two phases. This cycle then
  repeats with phase 1 starting at approximately 5.1 and again at 7.8
  ms.