I must say, the "medium vs. media" responses so far have been both more civil and more humorous than I expected!
Re: Medium left in the flash hole: When detonating primers put into the wrong cases, by putting the case in a vise and tapping the primer with a hammer and a nail-set (NOTE: ALWAYS use ear/eye/hand protection when detonating primers!!) I've observed that the primer flash extends an inch or so beyond the case for pistol primers, up to 2-3 inches beyond the case mouth for large rifle primers. Based on this observation, I agree that a bit of corncob or walnut would not be much of an obstruction for a primer being fired in a loaded case, in the chamber of a firearm.
BTW, there are other, much more effective methods of detonating unwanted primers.