Let's define "case trimming." I use that term to refer to reducing the length of a sized case. It looks like your'e talking about cutting down .223 cases to make 300 Blackout cases, and that's a different animal.
I have a jig attached to a mini-chop saw to shorten .223 cases.
I found it on 300BlkTalk, and it works great. After chopping cases, I anneal them one-at-a-time, and then size them. After they're sized, I trim them with my Giraud trimmer.
One member here says "the correct method" is to use a Dillon 1200 trimmer, which does save a few separate steps, but not only is that trimmer kinda expensive, it doesn't allow for annealing, which I believe helps prevent early case failure - and may make the initial sizing easier.
I didn't invent my process. It's based on the process dryflash3 presents in the
300 Blackout Master Thread in the
Reloading Forum. My variations are really just using a different method to chop the cases and a different trimmer for the finished cases.