Tromic, here is a thought for your consideration. The pneumatic speargun pistons usually have two or three rubber seals, a rear cup type seal and one or two "O" rings on the body of the piston. More seals mean more friction, but they stop the piston wobbling in the inner barrel. On the original late sixties model "Sten" the back-to-back dual rubber cup seal was very narrow and the piston tended to wobble, sometimes letting a blast of compressed air escape while the gun was being cocked. Mares at first overcame this wobble effect with a small plastic housing in the area of the sear lever that fitted inside the inner barrel tube, it stopped the piston's mushroom head from tilting on the sear tooth and stopped wobble at that position, but not when the piston zoomed off down the barrel or was being pushed back during loading of the gun. After that Mares changed to longer pistons in the familiar cylindrical format, usually with three seals, two cup type or cone seals at the ends and a mid-piston "O" ring.
Viktor Doroganich has built his unique pneumatic spearguns with a tapered body piston and only one rear seal. However this design requires tight control of the spear at the muzzle end to eliminate piston wobble as the piston only touches the wall of the inner barrel at its rear end. Normally you do not have this degree of muzzle control in a wet barrel pneumatic speargun, you only have a muzzle centralising washer for the shaft which is also the shaft stop ring. The vacuum barrel system does require a sealed slider, so this element can be used to control piston wobble. My suggestion is that you turn down the piston body to form a long taper and get rid of all the seals but the rear seal. These deletions will minimise friction in the inner barrel and also lighten the piston. One drawback will be that the tapered body piston will not be surfing along on the pressure wave inside a wet barrel if you use a Mamba type system, however it may not be a problem. Viktor Doroganich used a wet barrel because he adopted a hydro-brake muzzle on his speargun, so he needed the water inside the barrel. Also he could vent his muzzle with many relief port holes because it is made from titanium and screws onto strong stainless steel threads, the inner barrel being made from stainless steel. That meant that he did not have to worry about weakening the muzzle with so many relief port holes in a tight ring around the muzzle.
Attached is a photo of the Doroganich muzzle's directional control line slides. They fit closely on the shaft and are plastic "O" ring sealed at the rear periphery to control slop in the muzzle bore so that the line slides cannot fall out. There are no shock absorber springs or stop rings on the shaft, the spear tail stop diameter just strikes the rear of the line slide at the end of the shooting line's run. As the spear approaches the muzzle this frictional fit in the muzzle line slide (which is also made of titanium) is ejected by the pressure wave in the barrel, but by then it has done its job of directional control of the piston and spear combination.
A vacuum barrel pneumatic speargun could provide much the same function, so why not trim down the piston body and use the spear shaft to control its stability in the inner barrel instead?
You could do worse than to follow the lead of one of the masters of speargun design and fabrication.
