Not being a spearo I won't help you with the gun, but maybe the BCD freediving belt would be quite useful for you, because to certain extent it eliminates the need for leg propulsion, and helps you staying quite buoyant on the surface, while allowing for easy effortless descents and ascents. Having good buoyancy on the surface might help you with the loading too.
Check it out in this thread:
http://forums.deeperblue.net/general...reedivers.html
BTW, I admire your courage, and wish you successful and safe hunting!
