Yes, that's the cylinder I have and all my references state packed singly in a container. 
 
I suspect C320 got used for various things but it must have been introduced for something specific. As it's clearly almost identical to a C263, other than length, I thought 6 pdr APCBC but that round will fit in a standard C263 so who knows!
		 
		
		
		
		
		
		
			
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				Adrian Barrell
			 
		
		
		
		
		
	
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