I'm not a gun guy, but I think "shells" is more reserved for shotgun ammunition, or am I wrong? 9mm ammo is probably mostly called "rounds", I think.
I could be wrong, though.
Trust me, no expert anymore but I was very much into guns years ago.
I referred to loaded rifle rimfire, centerfire as well as shotgun shells all as “shells” when loaded.
May not have been technically correct, but it’s what I used.
I referred to spent centerfire rifle “shell” brass as “brass,” as I was into reloading.
A spent shotgun shell was still a “shell.” Same with a spent rimfire. I guess because I didn’t reload them.
Once the rifle “brass” was re-loaded with a primer, powder, and bullet again, it became a shell…. I tried to have about 500 loaded shells prior to each woodchuck season….
Might have just been my background/upbringing.
You often see the terms rounds, cartridges, and bullets (bullets not correct IMO), etc. I think it’s a matter of what you’re accustomed to.