Your long gun barrel seems to have deep rifling grooves.
A custom sizing die can be used to size a wide variety of bullets. And the outfit below can enlarge the size of a die if the diameter proves to be too small. That way the mold doesn't need to be the perfect size.
Lee makes custom sizing dies but has temporarily suspended most custom work because their business is booming.
But there's other outfits that will make them including this one:
A custom mold can also be enlarged to some degree.