Water, sealed over the nose with beeswax might indeed work on a HP or Hollow Cavity design --- maybe. If totally enclosed might be ok, as long as there wasn't an airspace to cause gyrations in flight. Fine line between full and too full causing it to stress break in the barrel due to obturation - hmmmm. t5hinking in terms of a soft lead in a ML- hard will not work due to the need to obturate to fill the rifling.

An interesting situation, but water would definitely start violent expansion. What about something slightly more compressible - would some sort of an oil work as in veggie oil - or it is as non-compressible as water?