Without some way of replicating the results, it remains an "urban myth", but this is supposed to have happened often enough that it's been called the "2.7 grain Bullseye surprise", because that is what the normal target load was that was supposed to give this effect. The problem is, that's a load that is SO light that it would be easy to miss a double or maybe even a triple overcharge, and, so many of these loads have been made and fired over the years, that if there is something specific about that load, it should happen with more regularity than it seems to.