The Collaborative International Dictionary of English v.0.48:

Hydropneumatic gun carriage \Hy`dro*pneu*mat"ic gun carriage\
   (Ordnance)
   A disappearing gun carriage in which the recoil is checked by
   cylinders containing liquid and air, the air when compressed
   furnishing the power for restoring the gun to the firing
   position. It is used with some English and European heavy
   guns.
   [Webster 1913 Suppl.]