The most common and most likely answer to this crossword clue is the 14 letter word UNDER-AND-OVER
U
N
D
E
R
-
A
N
D
-
O
V
E
R
Buddy explains!
The straight part
We know that "Two-barrelled gun" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case two-barrelled gun = under-and-over.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.