There is no underlined phrase in your question but the infinitive phrase is: TO FIRE THE NORTH KOREAN EQUIVALENT OF A SOVIET ANTI-CRAFT GUN
Answer: The infinitive phrase here functions a noun. It is the direct object of the verb “learned”.
Step-by-step explanation:
An infinitive phrase is a phrase that starts with an infinitive (to + a verb) together with other modifiers or objects.
Infinitive phrases can function as nouns or adjectives or Adverbs.
When they function as a noun, they can do it either as a subject or an object.
In the sentence above, it function as a noun and as the direct object of the verb “learned”.