Firearm part (6)
I believe this clue is a double definition.
'firearm' is the first definition.
(I know that firearm can be written as piece)
'part' is the second definition.
(I know that part can be written as piece)
(Other definitions for piece that I've seen before include "Musical composition" , "Morsel" , "Section; firearm" , "A bit of something" , "Unpleasant character" .)
(Other definitions for breech that I've seen before include "Bit of gun" , "Opening for loading a gun" , "(Half of) lower garment" , "Back part of a gun barrel" , "Opening in the barrel of a gun to load bullets" .)