Crossword Genius

A gunsmith will add it to his stock (6)

Ross

I believe the answer is:

barrel

I'm a little stuck... Click here to teach me more about this clue!

This could be a double definition.

'a gunsmith will add it to his' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'stock' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

Can you help me to learn more?

(Other definitions for barrel that I've seen before include "''Roll out the ..., let's have a ... of fun'' (WWII song)" , "Curved wooden vessel" , "needs some oil" , "VAT?" , "Container for beer etc" .)

I've seen this clue in The Globe and Mail.
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