The stored energy in a cocked to shoot hydropneumatic gun will be used to move the sliding masses inside the gun as well as propel the spear. Generally the masses are an annular piston carrying inner and outer "O" rings that seal on the respective tubing walls of the exterior of the inner barrel tube and the interior of the outer tank tube, such as is used in an "Aquatech" speargun; plus a releasing valve that controls the opening and closing action of the rear of the inner barrel. The releasing valve is usually small, so has a low mass.
In this new gun design the annular piston is now a combination of the inner barrel tube, the rear moving bulkhead (or sealed periphery disk) and the multi-grooved (rack section) moving front bulkhead that are all rigidly connected together to create one large assembly. Gun energy stored from muzzle loading effort has to drive this assembly rearwards when the gun shoots, which has much more mass than in guns such as the "Aquatech". Therefore less energy will be available for spear propulsion as to move these large internal sliding parts the energy has to come from somewhere.
I note that the spear tip head diameter is large enough so that the spear is not pushed right through the inner barrel as otherwise the water would exit the gun without a shot by running forwards around the spear shaft while it was still in the inner barrel, thus when that diametrical step hits the front edge of the sliding bulkhead that is the end of the loading stroke. You could pull the spear out and execute another muzzle loading action which would advance the inner barrel to another tooth position on the cylindrical rack, but that makes the gun longer each time you do it.
The rack on the sliding front bulkhead could pick up grit and scratch the bore inside the yellow part (front fixed bulkhead), maybe not a problem is use, but it will be when servicing the gun and dismantling and reassembling it, but that depends on how the gun actually goes together in terms of the order of assembly. A forward latching gun, but the latch is on the sliding inner barrel assembly, not the spear.
As hydropneumatic spearguns have thick wall tanks (the tank holds the gun ends together and therefore requires screw threads) the smaller models have little recoil, so not really an advantage to have a large mass flying backwards in the gun during the shot as a counter to the spear moving the other way.
Interesting idea, but it does not offer any advantages over the current designs and machining a cylindrical rack is a time consuming task and discards a lot of scrap metal for production guns, but maybe OK for a few guns on a limited basis.
A diagram is attached to help explain the points made above.
