spear size?

The heavier the shaft the more recoil you will have. Also more penetration, but you will need more bands power.
You must also consider that a heavier shaft will make your gun sink. It depends on the gun mass itself.

what are you hoping to hunt with this gun? i find that a 7.5 mm shaft is my favorite for over all use and for a smaller shorter range gun I like a 7mm shaft as it is a liitle faster.

to me, i would rather know my gun is not a speed shooter if I know the shaft will connect and penetrate..I am never upset about a missed shot, I am furious when one bounces off or fails to pebetrate..

for a three band gun you will not lack for power and i suspect you will be shooting bigger fish in open water...I would go w the thicker shaft

Quote from yellows

I'm fabricating a custom speargun and need help in deciding on shaft diameter.
The gun fits a 55'' shaft and will house 3 bands. I also plan on using a delrin track.
What the advantages and disadvantages to 9/32 or 5/16. Help.
Scott

In your avatar I see Mexico/ Cali we're not shooting foot long fish that dart the shaft. The 5/16 has more mass to pull 400 mono shooting line out 15 feet and still crash though the spine with a slip tip if you run 3 5/8 bands at 300 % elongation. Both will kill fish well, but my stack of bent 9/32 shafts is larger then the 5/16.

I do like the 9/32 for small local blue fin however, those fish have some James Brown shite going on.;)

Guys talk about WSB beating the shaft....but it has never happened to me if I keep the rubber fresh and short.

Cheers, Don

My opinion is that versatility is best. If you make the track and ballast the gun for 5/16" shaft you will still be able to shoot a 7.5mm (my prefered shaft diameter) as well as a 9/32 without adversly affecting the performance. Not so if you build the gun light and then want to go to a heavier 5/16 shaft.