The most common and most likely answer to this crossword clue is the 6 letter word FUSILS
I've seen this crossword clue in The Times
F
U
S
I
L
S
Buddy explains!

The straight part
We know that "firearms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case firearms = fusils.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 FUSILS mean?
FUSILS noun- A bearing of a rhomboidal figure, originally representing a spindle in shape, longer than a heraldic lozenge.
- A light flintlock musket or firelock.