DF,
I tried to search the web for a reference. No luck. Seems to be a coloquial term in the smaller circle of sxs gun traders and enthusiasts to denote a sxs (never saw a reference to a Model 12 30" duck gun as a "funeral gun") with all blue/black metal and possibly short barrels, typically 12g (maybe only due to commonality). Possibly, the same gun used in "shotgun weddings"? LOL Maybe someone else has some more specific history of the term.