A single bore sized "Punkin Ball" was the first "Slug", Oleg. Dunno about in rifled bores, but their performance in most smoothbores left plenty to be desired.
A Forensic Pathologist I knew a few decades ago was fond of multiple round ball loads for a 38 Special he had.
And, at one time a reduced 30 caliber small game load for 30-06,30-30,etc, involved a 00 shot swaged down a hair by passing it through a .308" die,and loading it over some 2400 powder(IIRC) and a Cream Of Wheat filler.
But, a 12 gauge bore sized ball runs a bit over an oz, so two of them qualify as an overload. Just one, with little bearing surface touching the lands, will strip easily, ruining accuracy. The short WCs mentioned in that thread deserve some attention, but it'd take a very narrow set of parameters to show them superior to standard loads and my gut reaction is that won't happen.
Standard ammo works very well. Even Tic Tactical and Altoids for Paranoids Loads will work(G). Why mess with success?....
