Marching cannon's mouth refers to a great cannon called a "field gun" that can be moved across the battlefield by several trained soldiers. To refer to this type of weapon in a more concise way, synonyms can be used such as an artillery piece, a howitzer cannon, or just a field cannon. Another synonym could be a piece of ordnance since they are used in military warfare. Essentially, synonyms for marching cannon's mouth would revolve around military jargon and weaponry, and it's important to have a good understanding of specific terminology to accurately describe this type of artillery.
