I stumbled upon one of these for making 300 Savage from 308 cases. But they have several variations for different conversions. It just requires a cheap Harbor Freight mini cutoff saw. I just set mine up and it seems pretty slick, it will save a lot of tedious trimming work. Here is some more info.

https://www.youtube.com/watch?v=ZrowjajNegc

https://elite3ddesign.com/collections/jigs-collection