Rapidly turn firearm, increasing core volume? (6)
I believe the answer is:
riffle
'rapidly turn' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for riffle that I've seen before include "Casually turn over" , "stir" .)