The most common and most likely answer to this crossword clue is the 7 letter word RELOADS
I've seen this crossword clue in The Guardian
R
E
L
O
A
D
S
Buddy explains!

The straight part
We know that "Inserts more bullets" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case inserts more bullets = reloads.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 RELOADS mean?
RELOADS noun- The process by which something is reloaded.
- To load (something) again
- To refresh a copy of a program etc. in memory or of a web page etc. on screen
- To load a gun again; or recharge a used cartridge.