The most common and most likely answer to this crossword clue is the 8 letter word ENFILADE
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result ENFILADE
Buddy explains!

The straight part
We know that "Discharge of firearms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case discharge of firearms = enfilade.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ENFILADE mean?
ENFILADE noun- A line or straight passage, or the position of that which lies in a straight line.
- Gunfire directed along the length of a target.
- A series of doors that provide a vista when open.
- To rake (something) with gunfire.
- To be directed toward (something) like enfilading gunfire.
- To arrange (rooms or other structures) in a row.