1. I would suggest that you contact the "retro sub" diving group in your area. They are very talented and make some great guns and or modify off the shelf items to be great shooters.
2. Most who produce their own shooten irons currently use selected components from the various manufactures...All though costly prefer the Alexander mechanism which is installed in a wood guns of my own design.
3. The home built guns are generally custom designed and made to the makers own exacting specifications, generally but not always they are more powerful, more accurate and more dependable. On rare occasions guns either shoot high or low at predetermined distances, this can be rectified by establishing an alternate aiming point or modifying the gun.
4. The story about Art & his brothers using an umbrella stay is noted and heard many times. A great story, many many things were used as projectiles and propellants in the geniuses of spear fishing by many many people on both sides of the US. However an umbrella stay would have the trajectory of a $5 dollar bill in a wind storm
sdm