I would agree that making a good rifled barrel would be more of a challenge than would be making a servicable receiver.
Read Spyktyr’s link from #21. It’s very interesting, and a relevant point that it makes is that barrel makers are almost always specialists, which suggests that it may not be something you want to try to do in a one off type setting. Almost all their machinery is special purpose and could only be used for gun barrels or other long precision cylindrical parts.