Another concept (though more complicated) in conjuction with the inserted barrel idea of Tony L (Can a break barrel ever be as efficient as a sliding breech springer) is to have a piston with a recess in the end which passes over the barrel which is inserted through the cylinder end wall, as the piston aproches the end of the travel it seals on the barrel outer thus creating a toroidal area of trapped air between the barrel outer and the cylinder walls/end to act as a buffer
It may be possible to set so as to stop bounce back and slow the piston relatively gradually, it could also mean that as the pellet is travelling up the barrel the air behind it is trapped though this may be a disadvantage as there may not be enough energy to keep it accelerating?
all my thinking is currently conceptual as i really dont want to do any maths I am not being paid to do