https://en.wikipedia.org/wiki/Puckle_gun

Puckle demonstrated two configurations of the basic design: the first, intended for use against Christian enemies, fired conventional round bullets, while the second, intended for use against the Muslim Turks, fired unconventional square bullets. The square bullets were considered to be more painful and would cause greater severe wounding. They would, according to the patent, “convince the Turks of the benefits of Christian civilization”.

Dunno if this is the one you’re thinking of, but it’s one example