The most common and most likely answer to this crossword clue is the 8 letter word HOWITZER
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "gun" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gun = howitzer.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 HOWITZER mean?
HOWITZER noun- A cannon that combines certain characteristics of guns and mortars. The howitzer delivers projectiles with medium velocities, by either low or high trajectories.
- Normally a cannon with a tube length of 20 to 30 calibers; however, the tube length can exceed 30 calibers and still be considered a howitzer when the high angle fire zoning solution permits range overlap between charges
- A powerfully hit shot.