One of my upgraded pieces of equipment was a new case trimmer. I got annoyed with having to keep adjusting the old one (Hornady trimmer) and keeping track of the collets was a pain.
After much research I settled on the L.E Wilson trimmer. I got a few extras that are options, one it the base and the other is a micrometer for easy adjustments.
This trimmer uses a specific case holder for each calibre though some can be used for more than 1 calibre if they are of the same family, 30-06, 270 Win and 280 Rem all use the same one. Each holder is shaped internally to hold the case straight.
The case is pushed into the holder and given a tap to make sure it is in.
Once you have set the micrometer to the desired length and done up the locking screw,
You then place the case holder into the trimmer and swing the clamp over to lock it in place making sure that the case is up against the shaft.
You can then turn the cutting handle to trim to your desired length.
Once the cutter has stopped cutting slide the cutter back, release the locking handle and remove the case holder. Give the case a gentle tap or two on the supplied block and the case will come free. Remember to check with a set of Vernier gauges to ensure that the case is the correct length. In this case the brass is 22-250 and should be trimmed to 1.902 Or 48.31 if going metric.
The benefits that the Wilson trimmer has is that due to the case holder and the two rails that it sits on, allows the cutter to trim the case mouth square. With the micrometer attached it allows you to quickly set the desired length and start trimming in a short period of time.
I found that the micrometer was pretty much spot on and only needed a slight adjustment to get it perfect. Once you get a system going you can get you brass trimmed quite quickly.
I found that it would be best to mount the trimmer to the bench or onto something to place in as vice as it tends to move around a bit requiring you to hold it in place all the time.