Here is another calculation using a "Work-Energy" approach. If the piston is travelling at 30 m/s when it hits the muzzle and it weighs 15 grams (0.015 Kg) then its kinetic energy is given by 0.5 x mass x velocity^2 which equals 0.5 x 0.015 x 30^2 = 6.75 joules. For the moment we will neglect the mass of the spear and the air mass travelling inside the gun. So to bring the piston velocity down to zero that energy has to be dissipated as noise and heat in the muzzle. Deforming the rubber absorbs energy, some of which turns into heat, but in water this heat is rapidly lost, so we do not notice it, but we do hear the sound of piston impact. Some of the energy is stored in the rubber sleeve and then returns the shock absorber back to the discharged gun position. Work and Energy are the same measurement units and work is defined as the force applied over a certain distance, say X, which let us say is the incremental movement of the shock absorber during the impact. With the gun discharged the piston is already leaning on the shock absorber with force 30 kg (using your figures), so initially the shock absorber is moved in by a distance D where there is a balance achieved between the force applied by the piston (under the reservoir's air pressure) and the resistance of the rubber sleeve to being crushed forwards to this distance D. When the shock absorber has moved to distance (X + D), during the shot, the piston has stopped moving forwards and the force on the piston is now 30 kg again (maybe a tiny bit less due to volume change with the piston slightly further forwards in the inner barrel), so then the rubber sleeve recovers and the stored energy pushes the shock absorber back to distance D where it always is when the gun is not cocked. The rubber sleeve is now in an equilibrium, so to speak, between rubber sleeve compression and the air pressure inside the gun.

If we add the spear to the equation then the kinetic energy is much greater as it is spear mass and piston mass combined moving at 30 m/s, but as the rapid deceleration commences the spear tail breaks free of the piston. So at some instant the spear and piston combined energy is going into the rubber sleeve, then it drops to only that of the piston. Now we know that spearguns of the same model have exactly the same muzzle even though they shoot different length spears, so either the muzzles are designed for the longest models with spare absorption capacity for the shorter ones, or absorption of any of the spear's kinetic energy does not really come into it. If it did then the spear would slow slightly while leaving the muzzle. Instead it is just the force to pull the spear free of the socket in the piston face which I think would be the same for all of the guns in that model series regardless of their length. The energy or work to pull the spear free is the force to do it times the distance that the force is applied over, however the distance is very short due to the tapered cone friction connection between spear tail and piston socket, so the energy used will be small as the distance will be small, maybe only a few millimetres.

Let us say that it takes 5 kg to pull the spear out, which we need to turn into a force of 5 x 9.8 Newton (9.8 conversion or 9.8 m/s^2). Say the force is exerted over 2 mm of travel distance Z inside the piston socket which is 0.002 metre, then the work done is F x Z = 0.098 Joule. Add that work-energy to the 6.75 Joule for the piston obtained earlier and we have 6.848 Joule.

Hence I think the energy absorbed by the shock absorber may be somewhat less than 10 joule, for this example anyway. If the gun was fired without a spear then the piston would travel much faster, the energy stored in the gun working only on accelerating the piston mass and not on the combined spear and piston mass, therefore with much greater terminal velocity. That is when the energy absorption capacity of the muzzle shock absorber may be exceeded and I know that such a shot will destroy the shock absorber in a "Seabear" breaking the polyurethane bush into tiny fragments that acts as the shock absorber in those guns. The plastic pistons and shock absorber bodies in Mares guns (and others) may be cracked by the experience, so I don't suggest anyone try it to find out what happens, in air anyway.