As a future reloader, what is case trimming and what are you trying to accomplish?
DT
Steve’s explanation is correct. Generally, you don’t have to trim brass every time. There are also ways that you can minimize how often you trim and therefore how long your cases last.
In the simplest case, with .223 brass, I can fire them 2-3 times before they need to be trimmed. Your mileage will vary depending on the rifle, chambering, the brass, and your reloading processes.
I could probably extend this to 3-4 times if I changed to other processes rather than full-length resizing every time, but given that .223 brass are cheap, I’m not inclined to go through additional steps to extend their life.
BTW, I use the Lee trimming system and a cordless drill as you can see from the picture. With this setup, I can consistently do 4 cases per minute, trimmed, deburred, and chamfered. As trimming goes, that’s the fastest cost-feasible system I could find. As John points out, the WFT system is faster and is certainly worth considering if it fits your needs. Since I trim a LOT of brass for 4 different cartridges and then I have about 5-6 more cartridges that I have to trim occasionally, the Lee system fits my needs the best because it is relatively fast and very cheap.
Grouse