I have also seen such things happen when one seats a bullet on a heavily compressed load. But this pic doesn't look like that's the problem........ I have seen the too-thick case neck problems appear using .308 brass necked to .l243 and not properly neck-turned after squeezing them down. But of course, when this happens, the whole neck is too thick, not just the bottom part into which the brass has flowed on previous shots. I know of no othe round that evidences such an effect to this extent-perhaps the .300 H&H and the .270 Win. ? I can see why some consider reloading the .243 Win. to be "touchy".