Firearm part (5)
I believe this 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 is a class)
(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" .)