I took a look at your capx. The gun rotating is just standard behavior for Construct 2, far as I know. Do you not want the gun to rotate at all or only to a certain degree (i.e, so the player's head can't all go Exorcist suddenly if the mouse turns too far)? Just want to be sure, as I've made some changes to improve your capx based on your issues, so input is needed.
Also, please do not create a separate thread for this, just keep problems per capx to one thread if you can.