So caliber has nothing to do with bore diameter?
Bore size is caliber. So the true statement wold 155mm x 58 = tube length
See 43.
Words often have several meanings/usages.
Calibers, plural, here means how many bore lengths a field piece has. It is the ratio of inner diameter to length, not the diameter, that matters.
One could have, hypothetically, a 155MM as mortar, as howitzer, and as rifle (gun). The barrel length would determine the classification, and hence use (range and trajectory).
Shotgun bores use a different caliber concept: A 10 Gauge bore would accommodate ten lead balls totalling a pound; a 12 Gauge would need twelve balls; a 20 Gauge would need twenty. (.410 is a rifled weapon caliber designation; it would be about 68 Gauge.)