Have an aeroprecision upper with a Criterion barrel, and aero R1 handguard. After approx 3-400 rounds noticed a slight rattle to the barrel. Turns out you can turn the barrel enough to bounce the index pin off of either side of the slot on the upper. No perceptable movement forwards but only rotationally. I took the barrel nut off. Pulled the barrel. Extension appears to be tight. Extension fit in the upper isn't excessively loose. I re-tightened the barrel nut to 55lbs.  Still the same issue.

What is going on here?

It could be perhaps the barrel nut is bottoming out against the face of the receiver before it gets pressure against the flange on the barrel extension.  Was the face of the receiver tenon "lapped" before assembly?

I didn't assemble, so not sure. But even if I weren't, wouldn't there be some play forwards/backwards?

Also would that mean that something has stretched or compressed, as it took some time before this showed up?

Could be enough to just not get tight, but still allow rotation.  Slide a piece of paper between the nut and the receiver and then see if you can pull it out after tightening the nut.