If my humble french doesn't betray me
			 
			 
			
		
		
		
			
			The 'barrel' would be the 'deposit/drum' for the rope hank; the orifice to tie the end of the rope. 
Such was the suggestion given by the original person; that this could be a fishing crossbow. The interpreation of the member quoted in the (here) posted images is a bit 'distorted'. 
I have lurked into this forum; several members giving wings to their imagination, like suggesting an incendiary crossbow, boar hunting crossbow and so on. One of them even made a draft on how it should have worked. The idea of a fishing crossbow with a rope (string) seems to be the more consensual (to them). 
 
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