I created the opening in this image using a sweep1. Is there why simple way of rotating the sweep’ed curve to create spirals (rifling)?
Original comment deleted.
“Infinite” option solves the problem I noted as Pascal describes below.
If you do use Twist, you probably want “Infinite=Yes” to avoid the twist effect falling off at the ends of the axis.
Yes, that is better. Removed the oddness at the lower left of the muzzle opening.