In a gun, the extremes literally are of brass (6)
I believe the answer is:
brazen
'of brass' is the definition.
'brazen' can be an answer for 'brass' (thesaurus). I am unsure of the 'of' bit.
'in a gun the extremes literally' is the wordplay.
'in' is an insertion indicator.
'a gun' becomes 'bren' (Bren is an example).
'the extremes literally' becomes 'az' (I can't explain this - if you can you should give a lot more credence to this answer).
'bren' going around 'az' is 'BRAZEN'.
'are' is the link.
Can you help me to learn more?
(Other definitions for brazen that I've seen before include "Insolent, barefaced" , "Bold-faced" , "Shameless and bold - resembling brass" , "Unashamedly flagrant" , "Shameless (impudence)" .)