Actually (presuming that this is NOT a "practical joke") a .905 caliber rifle would NOT be the largest bore rifle.
In the late 1880s or early 1890s. a gunsmith in (I think) Dublin built a black-powder CF rifle for a "Kenya-bound sportsman" in ONE & a HALF INCH bore.
W.D.M. "Karamoja" Bell said, with typical British understatement, that the "push-back" was "considerable".
(I wonder if ANYONE ever shot that rifle a SECOND TIME???)