My design on a rifled barrel for Nerf blasters. This is a 1/4 twist over 50mm. There are 4 rifling lines. One end is meant to fit on a 16mm barrel. The other end is to fit a 29mm long 9/16 inch K&S brass. 12 holes in the rifiling area allow for air porting. The idea is to give the dart a spin then let it stabilise before going off. This should reduce the number of stray shots.
The internal diameter of the rifling is 12mm and the inner diameter of the hole in that area is 14mm. The inner diameter of the part to fit the 16mm barrel is 16 mm and the inner diameter to hold the 9/16 brass is 14.6mm
I printed mine out using a 3rd party supplier, so I do not know the settings to print. But I usually request for 20% at 200 microns.