I have a 20" A2 configuration AR and wanted to play around with. I've got a 16" mid length upper hanging around and was wondering if this would function ok if I drop this upper on my rifle lower?
What sayeth the forum AR gurus?

Yes, it will function, assuming there are no issues with the 16" upper.

The length of the barrel in the upper has no relationship to the lower.

When I was stationed at FT Drum, I had to put my 16" BCM Mid length on a lower receiver with a rifle buffer, spring and tube. I had no issues with it shooting 5.56mm or .223 Wal-Mart special ammo.

Your mileage may vary based on ammo and gas port size, but based on my sample size of one it should work.

That's what I carry at work. As an aside, I've also put the same upper on a CAR style with a slidefire stock. No issues whatsoever.