155 mm is a NATO-standard artillery shell caliber that is used in many field guns, howitzers, and gun-howitzers. It is defined in AOP-29 part 1 with reference to STANAG 4425.

A field gun is a field artillery piece. Originally the term referred to smaller guns that could accompany a field army on the march, that when in combat could be moved about the battlefield in response to changing circumstances, as opposed to guns installed in a fort, or to siege cannons and mortars which are too large to be moved quickly, and would be used only in a prolonged siege.