Since a shot charge should act as a fluid when passing from the camber/hull through the forcing cone to the constriction of the barrel we would probably experience a venturi effect where by when a fluid is compressed velocity of the fluid increases (PA24 we should have considered this earlier but your comment on flying sparked a memory from my days studying elementary aerodynamics). I really don't think that the amount of velocity increase is enough to translate to a perceptible increase in felt recoil but we may have some recoil sensitive folks out there that can detect an increase in recoil of half a foot pound or so.