There are a variety of synonyms for the phrase "marching up the cannons mouth", which refers to boldly facing danger or walking into a difficult situation. Some similar phrases include "charging into the fray", "braving the storm", "venturing into the lion's den", "confronting the dragon", "facing the music", and "staring down the barrel". All of these phrases convey a sense of bravery, determination, and fearlessness in the face of adversity, and can be used to describe someone who is willing to take on a challenge or danger head-on, without flinching.