[Marwan]

well guns aside, i think you should be a role model for us here,you define passion for the sport.... happy hunting for you sir...
on topic, pneumatics might work if its a shorter gun, maybe a 50 gun could be loaded with one hand if you could support the gun on your waist/hip...
it seems you dive without a suit, am i correct, maybe you could keep using the same gun you are used to with shorter bands if you could stay afloat without having to paddle with your hands you could then load easily on the surface, breath through a snorkel. In your case maybe ballast diving could be advised... basically attach the weight to a buoy instead of on your belt, then after you load the gun on the surface where you will be floating(as you will be wearing a wetsuit with little weights, the rest will be on the belt attached to the float) hold on the weight and go down, leave the weight on the bottom to ascend easily, you wont loose it as it is tied to the float... maybe that could help... needless to say its a dangerous technique, so start shallow and gradually work up your depth comfort zone.
I would always recommend that you never go alone too, has nothing to do with your condition, none of us dive alone, thats the basic rule, so in that case you could also have your buddy load the gun for you...you would be taking a big risk in my opinion if you go alone...
happy hunting