There is a slight difference between "calibre" (caliber, take your pick of spelling) which is the diameter of the bore, and the "calibre length" of the barrel. The latter refers to the length of the barrel measured in terms of the bore diameter.
The length of the barrel of your 155 mm (about 6 inches bore) 39 calibre barrel would be 39x155mm, or about 20 feet. The 155 mm 52 calibre barrel would be 52x155mm, or about 27 feet long.
Another example is the Navy's 3"/50. 3 inches bore, 150 inch barrel length.
The distinction could be important. For example, the Bofors 40mm/60 was quite a different gun from the 40mm/70.
Barrel length is an internal measurement and does not include things like muzzle brakes, breech mechanisms, etc. It's not usually a useful measurement for modelers.
Cheers,
Doc