This should have been classified as a man-portable (?) weapon. Naval guns are classified as "rifles" because they ARE rifled! The largest "rifles" ever installed were the 18.1" rifles on the IJN Yamato and Musashi. At the US Army Ordnance School, there is a 36" mortar built for WWII. It was never fielded.
I haven't noted attacks by T.Rex lately, and I feel that a 20mm round would make mincemeat of one; at 1000m!! As an engineering exercise, it's interesting. As a weapon, it's as useful at a dragster is for transportation.