I've examined and shot the ACR.
Its running a short-stroke piston-driven operating system, this concept is combined with a multilug rotating bolt idea of the AR15.
A gas port located in abut the same location as the AR allows gas from the fired cartridge to flow into the cylinder directly above, within which is contained the head of a single spring-loaded piston.
The gas pressure forces the piston rearward, where is imparts force against a bolt carrier (more substantial then an M4), driving it and the rotating bolt head rearward. By rotating within a cam slot in the carrier, the bolt unlocks and carries rearward with the carrier, taking the fired case along for the ride until it clears the ejection port and is forced out by the spring-loaded bolt-face ejector. The bolt carrier stops at the end of its travel inside the receiver only (and not inside the buttstock as one would find in the AR), and is pushed forward by a guide and spring, stripping the next round from the magazine and chambering it.
The fire control system appears to use AR-compatible parts in the hammer, trigger disconnector and springs, with the exception of the hammer spring which as a angle turn on one leg to retain the firecontrol system retaining lever.
You can fold the stock on the ACR and there is a cheekpeice that can be raied for a much better cheekweld. Changing barrels is very fast on the ACR. Just a few things that I can think of that are significant advantages over the AR platform.
In the end - after my research, I did not prefer the ACR, it was the ergonomics that did not work well with me.
I went with the FN SCAR 16s. I also own multiple DI and piston ARs for comparison.