In some of my early experiments with case forming i did try sizing cases with a mandrel instead of just using a bushing. This is with turned necks. I turned 10 cases ( 6ppc ) to 263. And then after i fireformed the cases i used a 256 neck bushing to size the neck down again. Next step was getting the right mandrel to give me just the right feel when I seated my bulletts. Back then i was using my own bullets that where flat base and had a very small pressure ring 243.2. OK these are the problems i acquired using a mandrel. The first 3 fireings On the 10 cases worked out ok but after that I started getting very Unconsistent bolt closure. The reason the bolt did not have a smooth consistent feel anymore was beacuse of our little friend, yes you guessed it the dreaded donut. After a few firings that donut was moving up the case neck and by using a mandre I was pushing that donut out and by doing so I was getting very unconsistent bolt tension ,And as our friend speedy gonzales says, thats bad ju ju. So for that reason alone i was convinced not to use mandrels on turned necks. I guess on unturned brass it can work ok but for the type of shooting i do ( benchrest ) its not for me.
Gabe