Hello mate, been tempted into this discussion by your mention of theobens!
The gasrams they use are no easier to load. You have to use the same pressure as for a spring powered rifle as the pressure you put in is the pressure you get out. They are supposed to have a faster locktime, thats mainly due to a shorter piston stroke than springs.
I may well have this wrong but I believed pneuno spearing guns used air pressure behind the piston to power the spear? (Hence pumping into the rear of the gun increases the pressure/power). Airguns of course work by creating pressure infront of the piston once the potential energy of the sping/ram is released (ignoring pneumatic air-rifles, I'll come onto them). Which creates the most power? I don't know, though I can't imagine it would be possible to cock a full power air rifle spring with a straight pull as you do with spearing airguns....
I would love to know the power generated by a normal euro or pneumo speargun, does anyone know? I could work it out if anyone knows the velocity of the spear at the moment it leaves the gun (in air not water).
Now fully pneumatic guns, running from a compressed aircylinder which is part of the gun...I am amazed that such things are not already common! They can generate massive amounts of power, some big bore air rifles run at powers in excess of 500ft lbs, which is well into firearms territory (the legal UK limit for an air rifle without a firearms certificate is 12ft lbs). So compressed air could easily generate the power needed for spearing. My weihrach HW100 gets about 80 shots at full power per fill of gas, and that has tank no wider than a euro gun barrel and only 40cm long, so it should be possible to get plenty of shots from a speargun before it required filling.
So as Foxfish says a compressed air tank under a narrow spear barrel would surely produce a highly manouverable, powerful, VERY easy to load (no stretching of bands or straining to push in a piston as the power is already contained in the compressed air tank), and super SAFE speargun.
Anyway, thats my two pence for now!
Best shut up before I'm writing all night!