What I'm wondering is this. I've been thinking about getting a 300blk upper, because I already shoot and reload for both 223 and 308. I don't consider myself expert at either of these things, but I can get it done safely. At some point I plan to go AP and maybe pick up the pace a bit. My understanding with necked cartridges is, that a lot of the wear and tear is at the necked point, and when a case is no longer re loadable its often split at the neck or simply very thin there from being repeatedly fire formed and trimmed resulting in thing brass at that point. This understanding (and if I'm wrong, please let me know) led me to this thought that may not be anything new to more experienced reloaders.. What I am wondering is this. Starting with new brass in 223, could I be safe and reload the 223 ammo a few times (the number I could use the brass in 223 that would be nominally safe is info I seek), and then trim it down to 300blk length and reload in that caliber a few times or more? I don't ever push the envelope with my loads. I have so far found loads on the mild side to be more predictable for POI than hot loads, and probably less likely to wear hard on my guns. My planned 300blk loads will be on the "heavy bullet/low speed" side of things. Please feel free to address any fundamental misunderstandings my question may reveal, as my reloading is all self/book taught.