Speaking from an industrial brass tube forming background, the best way to get a consistent fit in the case neck on the bullet is by sizing the neck OD with a bushing down to where the neck ID is slightly smaller than the desired ID dimension. Then using a mandrel size the neck ID up to where the ID after spring back is the desired dimension. You may need to use a slightly larger mandrel to allow for spring back. This method is not reliant on consistency of the neck wall thickness to achieve the desired ID. So you do not need to be concerned with turning neck OD's to get a consistent ID. That said you may want your neck OD consistent for other good reasons like chamber fit and concentricity. This method is also better in the way expanding forces are applied to the brass pushing down toward the shoulder has more support, opposed to pulling away from the shoulder with an expander ball stretching the shoulder which is a leading cause of excessive bullet run out.
I too have loaded for many years in the traditional way with regular dies, then match bushing dies, and have turned necks as well, all have their place. But why I never thought about using a mandrel until recently when it started to catch on in precision reloading, I will never know. I just was not thinking of how we size tubes on machinery we build, and didn't put that knowledge to use in my reloading. I just did what the experts said to do, using what was being sold. I am very happy using mandrels now, and have had improved my ammunition, improving my rifles accuracy. Either way can acheive accuracy, which way is best? My vote is for the mandrel.
I too have loaded for many years in the traditional way with regular dies, then match bushing dies, and have turned necks as well, all have their place. But why I never thought about using a mandrel until recently when it started to catch on in precision reloading, I will never know. I just was not thinking of how we size tubes on machinery we build, and didn't put that knowledge to use in my reloading. I just did what the experts said to do, using what was being sold. I am very happy using mandrels now, and have had improved my ammunition, improving my rifles accuracy. Either way can acheive accuracy, which way is best? My vote is for the mandrel.