I am not sure that I am following what you said above. What is "guided muzzle"?
You are talking from the prospective of a "scuba" diver who is spearfishing not a free diver I think. In my part of the world, especially in Europe, spearfishing on scuba is a big no no (I don't agree or subscribe to this line of thinking) so not sure if what you said applies universally. The closed muzzles are made and sold in Europe and used by free divers not scuba divers. There must be another reason or benefit to closed muzzles in addition to what you said that applies to free divers. What I dislike the most about closed muzzle guns is that I have to use shafts with notches and a band with metal wishbone. Shafts with shark fins are too big to pass through the muzzle.