I've tried trimming long cases in my lathe, but it is time consuming. I use a round rod inside the case neck, and clamp the cases by the neck. Then I turn thee case off, and catch it in a pan sitting on the ways. I have a stop built, so it's easy to get the same cut each time, and I still final trim in my case trimmer.
But I stopped using the lathe when I saw someone using a miniature chop saw to rough cut cases. I got one on sale at Harbor Freight that has a 2" dia. blade, and it's so much faster! I simply set the stop for the cut I want, and chop the cases off. Then on the trimmer to make the final cut. I can cut them off faster than I could load them in my lathe.