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Absolutely! this is sometimes called blueprinting. I know civ-tac does this with their builds.

TexasJohnBoy wrote:I'm not familiar with lapping an upper, which area was smoothed out? Do you have any pictures? The only lapping I've ever done was on the bottom of heatsinks and water blocks for computers...

Basically the face of the front of the upper receiver is machined to make it sit exactly straight, and exactly perpendicular to the receiver. This allows the barrel to sit exactly straight and parallel with the gun. Without doing this, cheap machining could lead to your barrel being slightly crooked in relation to the upper receiver.