My method is probably kind of clunky, but I would create the shotgun with its pivot point offset behind the stock by a few pixels. Then I would use the Pin behavior to pin the shotgun to the player object.
Then you just make the shotgun set its angle to the position of the mouse pointer x and y every tick.
I've never created an example or published to the arcade before, so let me know if this is the correct way!
https://www.construct.net/en/free-online-games/pivoting-weapon-example-54683/play