Opening part of a firearm, by the sound of it (6)

I believe the answer is:

breach

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But I can't explain it at all! Could you help me understand?

(Other definitions for breach that I've seen before include "Rupture, rift" , "Gap in a wall or barrier" , "Non-observance" , "Opening - quarrel" , "Severance" .)

I've seen this clue in The Irish Independent.

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