Barrel is part of rifle (4)
I believe the answer is:
butt
This could be a double definition.
'barrel' is the first definition.
(I know that butt is a type of barrel)
'part of rifle' is the second definition.
'butt' can be an answer for 'part' (butt is a kind of part). I'm not certain of the 'of rifle' bit.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for butt that I've seen before include "Fag-end" , "Blunt end - target of ridicule - hit with the head - cask - US backside" , "Cigarette end" , "bum..." , "Contralto" .)