I try various stuff and, when in doubt, read the manual. My custom Biller, which I call the 'Carribean', is based on the Biller Limited, an all teak gun. Originally, it came with a simple clip to hold the line. I removed the clip and modified the spear to hold the loops of shooting line, in effect creating an automatic line release for the gun. I welded a small shark fin forward of the notches and directly over the drilled hole to which the shooting line is attached. You can see it in the article of the same name, posted on DB. Anyway, after testing various lines I settled on 1.5mm kevlar for the shooting line. The reel line is different. When the spear is released, the slender kevlar bolts through the gap formed by the muzzle and bands without any problem whatsoever. The loops drop away and pay out smoothly.If a stiff line is used it forms loops and whips through the gap. If cable is used one can actually hear the metal line whipping and lashing as it goes through the muzzle. Nothing bad happens but the kevlar is just so nice for this particular gun. Always keep an eye out for fraying, though.