As a rule for south Florida spearfishing I use 7mm shafts. I find 6.5mm shafts to be faster but they also lose momentum quickly over a short distance, resulting in not enough penetration on bigger fish. Using 7.5mm and 8mm shafts the speargun feels a little heavy, and they're harder for me to straighten if bent. But there's a right time and place for all shaft diameters. I want to share some loosely relevant experiences that may educate and influence someone's choice of shaft diameter.
I was using a 7mm 170cm Rob Allen carbon steel shaft with a 120cm Speardiver Phantom carbon speargun last weekend diving out of Pompano beach. Diving in about 45ft, clear visibility and a stiff current. I couldn't equalize on this day resulting in poor performance on my part, I couldn't even reach bottom. Saw a nice jack on the bottom from the surface. Tried to push through the pain to get to him but ended up more than 10ft away above, still took the shot, missed, and the shaft was stuck in the reef. Returning to the surface I was instantly swept away by the current with little hope of getting back on top of the shaft, much less being able to breath up sufficiently to get to the bottom and extract it.
My only hope to not lose the speargun was attracting the attention of the boat driver which fortunately I was able to do. In the meanwhile hanging on to the float, which was connected to the gun with a float line, I put significant pressure on the shaft, current was approximately perpendicular to the line of the shot. Boat picked me up, I sat on the transom holding on to my float and directed the driver to move the boat in the direction of my best estimate to end up behind the line of the shot, hoping the shaft will pull out. This would be the best case scenario, next best would be to cut the shooting line and leave the shaft on the bottom, but I was doubtful I could reach it in my condition. Anyways the float line tightened and I was hoping my crimps will hold. After a tense moment there was a pop and a release. I quickly reeled in the float line. The first relief came when I saw the white handle of the gun coming into vie, next a happy moment when I saw the mono shooting line was weighed down. When the shaft came into view I did not see a flopper. When I got it in my hands I realized the point has broken off at the flopper pin hole. I was amazed the 300lb mono did not break or slip through the crimps, it ended up stretched and a little loose over the line release but was perfectly usable. I think this is a testament to the quality of Speardiver monofilament shooting line.