Awards on AC metal are bookable using UA or US miles under the Star Alliance awards. I've had good success using US miles for AC awards; however, UA does practice "StarNet Blocking" which is basically making partner awards unbookable at certain times, the theory being to drive the customer towards using UA miles for UA metal at higher rates. And AC like all carriers holds back a portion of award seats, to be bookable only by their customers using their miles (which I firmly agree with; you have to take care of your own folks first). So theoretically you can use your UA miles for an AC flight or an AC/US mixed itinerary but there's still the question of if it's actually bookable through UA or not.