Dullard shows calibre (4)
I believe the answer is:
bore
I believe this is a double definition.
'dullard' is the first definition.
(synonyms)
'calibre' is the second definition.
(both can mean the hollow part of a gun)
'shows' acts as a link.
(Other definitions for bore that I've seen before include "Dull person; use a drill" , "Supported; dull person" , "Tedious person - wave - make a hole" , "Person who talks tediously" , "Fast-running water" .)