The most common and most likely answer to this crossword clue is the 7 letter word GUNNERY
G
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The straight part
We know that "use of firearms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case use of firearms = gunnery.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 GUNNERY mean?
GUNNERY noun- The science of guns and gunfire including aspects of bullet flight and impact.
- The design and manufacture of guns, particularly those of a large caliber.
- The firing of guns
- A place where guns are tested, or where people are trained in their use