Prospector makes an interesting point about just tying dyneema reel line to the shaft so that the reel line IS the shooting line. I know that many people do that, but like so many things about our rigging, it depends on the size of the fish we are shooting. Just yesterday I personally saw an example of the tradeoffs.
A couple of years ago I quite using reels and went to breakaway float lines. I don't want to start a debate here about the relative merits because among California divers that can be almost as explosive as religion or politics, but the main reason I switched was that a thick vinyl float line is much easier to grab to put pressure on a fish and a lot easier to follow down through the kelp in poor visibility after a fish wraps up in the kelp on the bottom. At age 79 I have to make it as easy as I can.
Which brings me to yesterday. My sinuses wouldn't clear so I had to get out of the water. My dive buddy wanted to try my Abellan Denton 120 so I handed it to him and wished him luck. He also uses a float line. For many years I've been using 2 mm 400 pound mono shooting line, but recently I've read about some pool tests of various lines and have switched to this 1.4mm stiff dyneema.
https://benthicoceansports.com/collections/lines-rigging/products/1-4mm-stiff-dyneema
According to the tests it causes a lot less drag on the shot and therefore improves range and accuracy. It also is a lot more resistant than mono to cuts and nicks. And unlike other softer dyneema braids, it can be crimped just like mono. So back to yesterday.
My buddy shot this nice white sea bass from the surface at a range of about 2 feet from the tip. It ran for the bottom and he grabbed to shooting line to try to stop it. It cut his glove and his hand and the fish got to the bottom anyway. When he got back in the boat he said he liked the gun but he didn't like my shooting line. I reminded him that we are usually grabbing our float lines to try to stop a fish. Its unusual to shoot a fish from such close range and then be able to grab the shooting line before the fish has taken it.
OK, now I'll finally get to the point. If the same line was used for reel line and shooting line, then we would always have to grab something that would cut our gloves and hands. So maybe the solution is to use thicker line. When I used reels, I used 3mm spectra because I wanted something thick enough to grab, but that would be way too thick for shooting line and would seriously retard the shot. So how about a compromise? On that Benthic site above, notice that he also sells 1.7 mm and 1.9 mm stiff dyneema. Those same pool tests that recommended the 1.4 mm said that 1.9 mm caused a reduction in range and accuracy and for me that would still be too thin to comfortably grab to put drag on the fish.
So if you are still with me, my conclusion is that if you are trying to shoot large fish, using your reel line to tie to the shaft as shooting line won't work. The line will be either too thin for reel line, or too thick for shooting line. On the other hand, if you are shooting small fish it probably isn't a big deal. Don't ask me where the cutoff is between big fish and small fish.
Isn't this kelp pretty? The problem is that fish get the line all wrapped up in it and we have to dive to cut them out.