I use a Mamba 90 & that is difficult enough to load! What diameter spear are you using? What pressure is the gun pumped up to.
I manage OK now I have had some practice. There is a lot of technique involved but it is a lot easier when you can stand on a firm setting. As I purposely brought my gun to hunt in shallow water & amongst reefs I can usually find somewhere to stand up out of the water & re load. You need to somehow brace the gun butt on your foot, place one hand on the middle of the spear shaft & use you other hand on the loader.
My best advice is to "join the dark side" & buy a Mamba 90, awesome gun
