"March up the cannon mouth" is an idiomatic phrase that refers to putting oneself in a risky or dangerous situation. Instead of using this phrase, one might consider utilizing synonyms such as "put oneself in harm's way," "tempt fate," or "play with fire." Other alternatives might include "venture into the unknown," "take a gamble," or "ride the edge." Each of these idioms conveys the idea of taking a chance or exposing oneself to potential danger, but they offer more descriptive nuances in terms of the specific nature of the risk involved.