...X2 is barely big enough to almost complete an AR lower 80%...
have to take exception to that statement, as would dozens of folks that have finished 80% AR lowers on X2 clones.
there are ops that can't be done on such a mill, but the current commonly available flavors of 80%'ers do not require any of those.
X2's are every bit up to the job.
i'll agree with the stuff about an X3 clone being preferable, and the ± of round-column mill-drills.