It's actually based on the HK/M416 (which is itself an improved derivative from the M16), which I think will replace the M16 as an assault rifle.
It's also one of my favorite weapons in Battlefield 3, since it has a 45-round magazine, is much more accurate than the M240B/M249 SAW, and has a faster reload time. But that's just a game, and I'm not a Marine, so I can't tell you what's best for the support gunners.
@unr3al: The ACR doesn't really look like an M16, although it's got a similar shape, since it was based on it and was heavily modified by Masada. Neither does the SCAR BTW.