Actually, yes, they are interchangeable.
I own both pistols and have interchanged them after doing all the external measurements/comparisons. The barrels are identical in all pertinent external dimensions. They are not however exactly the same. The G36 barrel has more aggressive lateral throating to the feed chamber where the feed ramp enters. This does nothing to prevent the barrel from being put into a G30 but might affect reliability.
As far as the reliability of an aftermarket G30 barrel in a G36 I have no idea. My guess is that (given the typically tighter chamber tolerances of)the aftermarket barrel might pose feed reliability problems in a G36.
