A very simple rifled barrel.
This could be used for anything. A prop for a realistic gun barrel, or a dart gun barrel coke; to give a soft projectile spin.
I printed this out PETG, but you could use anything for your projects.
Inside barrel measurements
bottom of the groves 17.90mm
top of the groves 16.78mm
Outside barrel measurements
diameter 19.64mm
Length 101.00mm