The largest sporting rifles ever actually used in the field as more than an exhibition were 4-bore stopping rifles, firing roughly 1″ in diameter (25mm) projectiles. These were intended to not simply kill a dangerous animal, but to stop it immediately in a charge, which might require shooting through thick bone or horn protection. For that purpose, the 4-bore could have some value – assuming you had an assistant to carry it when danger was not imminent!
However, there have been a number of gunsmiths who have built 2-bore rifles, like this example named “Double Deuce” by its maker, Stolzer & Son of Kansas. At 44 pounds, this behemoth is definitely too heavy for practical use, but it sure is an impressively huge piece of work! It fires a 1.33″ (33.8mm) ball weighing 3500gr (227g) at about 1250fps (380 m/s)…if you can hold it on target long enough to get an accurate shot.
You can check out Stolzer’s YouTube channel for a whole slew of videos on the making of these two guns, and more.