The AC556 is different from the Mini14
How so? For one thing, in terms of dimension, the receiver is completely different. The AC556 receiver not only has the external selector switch, but in length is slightly longer than the Mini14. Thus, with the exception of a few parts (safety, safety spring, hammer plunger and hammer spring), the trigger group of an AC556 will not even fit on a Mini14.
The AC556 trigger group has additional parts not only for the full auto feature but for the 3 shot burst feature. On the three shot burst, if you only fire two shots and then stop, your next burst will only be one shot.
While the barrel isn't necessarily heavier on the AC556, the gas port may vary in size depending on the barrel length with the general rule being the shorter the barrel, the bigger the port.