Although I've been reloading for nearly 30 years, I have never gone to the lengths of turning case necks.
I am workiing with this concept now and have a question.
Is it better to turn the necks after resizing the case, or after firing it. And why?
The mandrill supplied will not fit into the resized case, but I can turn it down if neccesary.

Turning after sizing makes sense to me but as you found out the mandrels are made to fit fired cases at least with Forester.

I would do it as then you can turn new brass also and just size a neck first that you wanted to turn. Just clean them up and don't take too much off.

First of all, turning necks in anything short of a tight necked competition chamber is usually unnecessary and counterproductive to accuracy. I believe "the man" on this site has done it and posted negative results. Those results tally with my experience and most others who've tried it. If the neck walls of the brass are uniform to within .0015" there is usually no need to turn. If they are not, buy better brass. It's usually cheaper, better, and not as much work.

Where turning IS necessary, either for tight necked chambers or for wildcats where the new neck is somewhere on the old body of the parent brass, it's usually best to turn AFTER the initial fireforming and with a carefully neck sized case that is expanded to get the irregularity of the neck to the outside. Expanders are available from Sinclair and other suppliers to conform to the mandrell used for turning. Some are available from RCBS which can be polished down on a drill press to more closely conform to the mandrell of the turning tool. Another alternative is to use a reaming die, also available from RCBS and others, which smooths the inside of the neck while it is supported by the die on the outside.

Bottom line is that you will be better off to dump any brass that isn't uniform and buy a lot that doesn't require turning. In my experience of near 50 years of reloading I've never seen any advantage to turning except as noted above.

Something else to consider is that while neck turning brass will give you more consistent tension on the bullet, it will be a weaker tension with respect to non-neck turned brass and may limit your ability to reach the pressures you want.

Benchrest shooters aren't bothered by this as most typically seat their bullets into the lands to insure alignment with the side benefit of holding the bullet while pressure builds..

It is a poor practice to turn necks unless you are shooting a varmint or target rifle with zero tollerance necks...It was never ment for oversize standard chambers and is definately counter productive..

It has always seemed to me that at least trueing up case necks should help accuracy, even in standard chambered rifles. I have tried it with any number of calibers over the last twenty years or so, and have yet to see any improvement. A total waste of time, it seems, though I will probably tilt at that windmill again some time.

Hey! I just read this thread and thanks for the insight!! Of course I read it after buying a lot of 100 cases and neck turning all of them!! *&%#@$#@&!!