It looks like the holster is holding a 40mm pistol launcher, like the airsoft ones knocking about but with a custom grip etc. Probably a more "realistic" take on a grapple launcher.
Have you tried looking at stainless steel wire rope cable. Strong, flexible, has a good look to it. I reckon 0.6/0.8mm would work well. It's pretty cheap too. I found some on ebay for about £9 for 50m. I mean you won't be scaling buildings with it but it's better than string.
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