It is really not a very good idea for an inexperienced diver, particularly not with a 30kg weight.
Coming up fast is not a problem on a freedive, but going down fast can be. If you have the weight in one hand, and the gun in the other you will have to be pretty amazing at hands free equalization to cope with the speed of descent from a 30kg weight.
I dont want to sound patronising Geoff, but you always seem to want to find a mechanical solution to issues that just need more time in the water. There are no magic shortcuts or silver bullets in spearfishing, time in the water and gradual progress is everything. If you cant fish at 15m, fish at 5m (or whatever depth you are comfortable at), gradually you will find your bottom times and depths improve. Likewise, if you cant get close enough to the fish, it is better to concentrate on being smoother and more stealthy in the water and learning about fish behaviour, rather than trying to use a huge speargun that you cant load without another Heath Robinson device
cheers
dave
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