The German "Paris-gun" was originally a 211mm bore with a 28 meter barrel, making the barrel approximately 133 calibers long. It was later bored out to 238mm, reducing it to 119 calibersI'd like to see some high-speed video of the barrel harmonics on that 88" long tube! I'll bet that it whips around like a wet noodle! The Germans (and probably others) made some cannon tubes so long that they required a trestle to minimize barrel droop. IIRC, the infamous 'Paris Gun' was one such.
Palma shooters at least used to prefer longer than average barrel lengths. Seems like 28-30" were common on those guns. That may have changed in recent years.