When I was a kid and wanted to get into spear fishing (free diving) I made something similar that we called a Hawaiian sling.
It was a tube that the the spear went through with surgical tubing attached to the tube and bent piano wire at the end that knocked into the spear.
Hold the tube in one hand and pull back the spear with the other.
Effective range in the water was pretty much limited to the length of the spear so you've got to get close to the fish but that's not as hard as it sounds somewhere where people don't spear fish regularly.
I only had limited success mainly due to cold water in the UK but when you're a kid that dosn't mater.
Now I know more about rubber powered projectiles you've gone and put the idea back into my head (possibly a dangerous thing :naughty: )