I've got a new SBR-registered lower.

It's labeled .223, but that's not supposed to matter.

Any suggestions on an upper?

I bought it with 9mm in mind, but technically I've ALREADY got a 9mm AR that runs 100%. It's the Olympic K9GL. Accepts Glock magazines, of which I already have plenty.

I just so happen to have most of the spare-parts laying around to do a 223 build, but I'm not entirely sure why I need a short .223.

6.8SPC is supposed to be especially compatible with short-barrels.

There's always 22LR. With ammo prices what they are these days, seems like you can never have too many 22's. With that said though, my new Nordic AR-22 seems to have this venue covered.

300 Whisper maybe?

Mike