I have a 7mm carbon steel south African shaft that I adapted for use with a pneumatic speargun. I chose a shaft that's longer than what is required for the 110cm air gun, so I was able to cut off the rear end with the loading notches leaving a straight shaft with the flopper already installed. Shaping the back end to fit the piston is quick work with a bench grinder. But drilling a hole for attachment of the mono shooting line in these extremely hard shafts is no simple task. I found the only way it can be done is with a carbide drill bit. If you can't find a carbide drill bit you can also use a 1/8 masonry bit with a carbide tip.
I'm left with two unprotected areas on the shaft, the rear end and shooting line hole, that I know will start rusting immediately. I don't know the exact process used on these south African carbon spring steel shafts to make them rust resistant. I'm guessing they are galvanized. So I looked for information on how to galvanize at home and found this video. I hope it really is as simple as demonstrated.