What was the largest-calibre rifled weapon ever used in combat?

I wish rulers and governments would put as much effort into saving people and our planet as they have in warfare. Just think of what could be accomplished if all the military budgets were used towards something positive and beneficial.

Tom, yes that's right

I am not a number., I am guessing it wasn't rifled. Ball shot never is.

The picture is of the Tsar Cannon produced in 1586 in Russia. It would fire stone balls and was also larger than the Schwerer Gustav at 89cm (35") but it's uncertain whether it was used in combat. The explanation only mentions other explosive shell cannons.