The expander mandrel method has been around for a very long time. I like it for several reasons.
When I was young and couldn't afford to blow big budgets on brass, the availability of military surplus was a great way to get your brass, but it came with the burden of more prep than starting with your own once fired cases. Many of them would have dented necks for example. We got a much better yield from a batch of brass when we ironed out those necks from an expander mandrel than any other method. Having the exposure to the mandrel for the purpose of dressing up surplus brass made them an easy choice when it came to more advanced accuracy techniques later on.
We often needed to normalize a process by getting all the different brass neck thickness to the same standard. We would start brass by punching out all those crimped in primers and prepping those primer pockets. The brass was then cleaned to look almost virgin. The next step was to run the expander mandrel on the ones with dented necks to get them ironed out to roughly start in the sizer dies. We pulled the expander buttons out to reduce the working on the brass, so some were sized more than others. We then used a sorting method where we expanded the necks to run both a trim and neck turn operation. Those operations required a fairly good tolerance on the inside of the necks to make sure they would process well on the pilots for the trimmer and neck turners.
From then on, all the brass had the same neck thickness and we could use either one piece sizers or bushing sizers, but we didn't run expander balls because they were harder to control. With expander mandrels, we could step the madrel sizes to give us the finished neck diameters to within tenths. With expander balls, we just didn't have the ability to keep enough expander balls at the diameter we needed, but that was just because our cutter grind friends could easily make us a set of expander mandrels. I suppose an expander ball set could be done just as well, but the expander mandrels were easier to run since they don't have to be in the die ahead of time like the ball.
Once you have a set of expander mandrels in fine increments for a given caliber, you can process brass with fine control and less cold working of the necks. Over the long term, there is a benefit to case life as well as fine speed controls from neck tension.
BTW, another tip I try to give young folks who ask about advance reloading... is to watch surplus sales for sets of DelTronic pins in the sizes of their target neck diameters. By having fine increment gage pins to measure the effects of your sizing process, you can learn to get the neck tension and batch controls faster by not having to struggle with waiting to shoot to see if you got it right.
