this looks like a bad load to begin with.
there are two pressure fronts, both with significantly different burn rates.
the first casual comment would be this is is a really bad duplex load but no, i'll bet we are seeing the bullet being moved by the first pressure front, then the pressure drops but the remaining powder, slow burning indeed, burns at a much slower rate at the lower pressure until the surface retardants are consumed then you end up with large geometry grains burning rapidly giving you the second pressure front
the second pressure front is steeper, faster, taken to the extreme, this load will have a detonation at the second pressure front instead of a regular burn. this is a bad condition where the powder evolves from solid to gas with a flame front of greater 5-10kF/S. pressure skyrockets, metal yields to the energy and your gun grenades.
this i have seen before, when you use powder that is too slow for the case volume and bullet weight, or when you use small amounts of slow powder, or, greatly reduced charges of fast ball powders.
but, this is all a bad and potentially dangerous condition, not one you want to load for regularly.
properly chosen powders, case volumes, and bullet weights will not cause this anomoly.
although, this could also be a barrel obstruction !!!!! so much for being analytical !!!
and yes fred, i do go after grouse with nothing but 1/2 ounce in my 410 !
sg
