As the term elephant gun does encompass several types of guns, I should have perhaps given an example, rather than just a link to Wikipedia.
I don't mean the later high velocity or "express" hunting rifles, I meant the earlier muzzle loading oversized blunderbusses.
The sort of weapon that in comedy films the Victorian Explorer fires and is thrown onto the ground or back 10ft feet or more (usually in a large cloud of smoke).
My design thoughts were that it was an oversized shotgun, firing a large single projectile, with a kick like a mule.