But won't a piston cannon of that size need to roll back so the recoil is dissipated? It might be too much stress for the fittings.
You are thinking in terms of explosive ordinance. When you analyze an air cannon, All the pressure is already there sitting.. Waiting..
It's already holding all the peak force that it can deliver. All we are doing is opening up one side of the pressure container to eject a projectile. You do want to spread out the projectile force, but you do not need a spring recoil system.