The barrel end of a gun (4)
I believe the answer is:
butt
I believe this is a triple definition!
'the barrel' is the first definition.
(butt is a kind of barrel)
'end' is the second definition.
(I've seen this before)
'of' is a link.
'a gun' is the third definition.
(thesaurus)
(Other definitions for butt that I've seen before include "Hit with the head; object of ridicule" , "Cigar end" , "Scapegoat - strike - bottom - barrel" , "at 16 down [MAN UNITED]" , ""bottom" to US" .)