I have a Chiappa upper on my AR, it shoots pretty well (I made pretty tight groups on 50yds) and CCI mini mags ammo works just fine. The only thing I don't like is that you can't remove the charging handle, but it's no big deal.
I also have another almost new Chiappa upper for sale if you are interested. It is listed on GunBroker.
personally i would go with the cmmg. ive got their plain kit it works flawless. simple design easy to repair if you should ever need to. i would think their dedicated upper would be even better. a future want for me is their govt profile barrel dedicated for 22lr
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent