The most common and most likely answer to this crossword clue is the 8 letter word ARQUEBUS
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More clues leading to the result ARQUEBUS
Buddy explains!

The straight part
We know that "Long gun" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case long gun = arquebus.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 ARQUEBUS mean?
ARQUEBUS verb- To shoot with an arquebus.
- An obsolete matchlock firearm.
- A portable gun, varying in size from a small cannon to a musket. When used in the field it was supported upon a tripod or trestle.