I think nearly all builders who make MLers for a living would be able to construct a LR from what is described as "from scratch".....but supporting oneself and family in the gunbuilding business doesn't financially allow it. Even many of the original builders bought the locks and bbls as many of us do today. Also bought by many original builders were brass Bplates and TGs.
Making bbls requires a forge and rifling machines, etc, lock making from scratch requires a lot of time and TGs and Bplates require casting equipment and patterns.
If today's builder buys the bbls, locks, Bplates and TGs, builds from a blank and makes the rest of the parts, he emulates many of the original builders. ......Fred