Shell and gun taken from volatile youngster (6)

I believe the answer is:

oyster

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'shell' is the definition.
(oyster shell is a kind of shell)

I can't explain the rest of the clue.

(Other definitions for oyster that I've seen before include "The world of Shakespeare" , "Succulent bivalve, may bear a pearl" , "person unwilling to communicate" , "taken in by carpenter" , "Marine bivalve" .)

I've seen this clue in the Sydney Morning Herald.

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