I've seen a few in Iraq during the initial push, but that was the only time.
As to the M16/M4 combo, know it well.
Was issued for a few months a M4/203, and one of the Mortarmen in the troop hated trying to fit a full sized M16 in the Mortartrack. So... we swapped lowers. The M4/203 with a fixed M16 stock is perhaps the best balanced rifle I ever carried in the Army.