Board Thread:Questions and Answers/@comment-112.206.128.133-20160415134638/@comment-71.83.236.251-20160428030031

This is from the Servant page on "Humans and Modern Weaponry"

As spiritual beings and divine mysteries, Servants cannot be harmed by normal means and are still considered spirits even in physical form, so they cannot be physically harmed by regular humans or modern weaponry. They can only be harmed by attacks accompanied by magical energy like a Servant throwing dirks imbued with magical energy, so they can ignore even debris in battle unless it should become contaminated by an enemy's magical energy. Only Servants can normally harm other Servants physically, so even a paper knife could be used to harm a Servant should another Servant wield it.

A Servant does not require a Skill or Noble Phantasm for the random weapon to be useful. Servants are generally hacking away at each other's prana / magical energy, which is simultaneously their 'health bar' and their 'mana bar'. However, a general prana-embued weapon would probably be inferior to anything a Servant is summoned with, even a nameless armament. It would especially be inferior to a mini-gun turned into a Noble Phantasm, done by Lancelot's Knight of Honor.

Something else to note is Mana-Burst / Prana-Burst skill. It's a skill specialized in embuing prana into an object, but over-embuing would destroy the object. It raises the power of ordinary weapons (the paper knife) but the weapons cannot handle it.