Dan,
I think it was a pressure problem. But not knowing all the facts we'll both keep wondering what really occured. Now as the crossing at 20 yds. one of two things is in play. First the rifle came that way from the factory(don't think so though). Or the wedge at the muzzle has come loose. This can and does happen most of the time from excessive pressures with hand loads. But in any case the stretching is definitly excessive pressure what ever the cause. And you know as I do that very few handloaders admits to screwing up, especically when they're not aware they screwed up.