Someone just did an article about barrel lockup (brownells, maybe?). They had a loose fitting barrel in a factory gun and wanted to see if it was a factor. Put gun in a Ransom rest and shot groups. It shot very well. Next the tester pushed on the barrel horizontally, moving it to the end of the "wiggle". Rounds flew a few inches off to the side. Pushed on it again before firing, same spot off to the side. Next he pushed on it in the opposite direction, same size group, but in the other direction. He concluded that the lockup was not as important as the barrel returning to the same spot after each shot.
Keep in mind, this was with a rest. When you introduce running and quick movement, I imagine that it's possible the barrel can move out of it's "resting" position.