Speargun Rubber: Spearguns other than pneumatic spearguns are powered with rubber. The rubber is pulled back and hooked into the notch of a spear by the bridle on the rubber.
The thicker your speargun rubber is the harder it will be to load and the more power it will give. Alternatively, you could have two thinner rubbers that are easy to load but still provide maximum power. Twin 16mm rubbers are ideal for most kinds of spearfishing unless you are hunting marlin or large tuna which may require several power bands.