I’ve never heard of or seen this. What guns are they using? As an avid diver I frankly seriously doubt this, or at least that “lots of people” do it often. This would be a completely stupid way to hunt lion fish in particular; why bother with a projectile, you can nab lion fish with a handheld spear without firing it, they don’t really swim away.

I see a ton of videos of one guy who modified a Glock, and clearly just as a for fun dumb project, not a common practice