You probably need to explain the your logic a little better with regard to the relationship between the neck turner and the Lee collet die. If you're asking whether the mandrel in the collet die can substitute for an expander mandrel before turning, then the answer is no.
Two reasons - the collet die will not establish a clean neck/shoulder junction fo you to set your neck turner against, and the collet itself will leave some shallow ridges on the outside of the case, which will get turned off. This will result in thinner "streaks" running parralel to the neck along the length of the neck. Also, the collet may not reach all the way to the shoulder, so when you turn the whole neck to a consistent OD, the lower protion will be thinner than the upper portion.
The preferred method is to F/L size with the expander ball out of the die, then run the case over an expander to bring the ID up to a tight fit on the turner mandrel while turning.
Elkbane
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