I don't know but I am surprised nobody replied yet. You could try contacting the distributer (Florida Freedivers in the USA last time I looked) or railgun specialists Sumora, or RA direct via their website
Rob Allen spearguns, railguns and other spearfishing and freediving accessories.
I normally find heat helps free stuck things (as the longer external part usually expands faster than the internal part). Maybe soak the joint with warm water after removing the barrel screw? It might also help dissolve any salt that might be present. You are dealing with polymers, so I wouldn't apply too much heat.
Why remove the barrel though?
