Thunderer

is the Noble Phantasm of Billy the Kid. It is Billy's , also known as Thunderer, coupled with his signature counter-like triple-burst shot, sublimated into this Noble Phantasm.
Strictly speaking, Thunderer is not the gun itself, but the concept of the gun being fired by Billy the Kid, so it is more like a Personal Skill than a Noble Phantasm when viewed in such a comprehensive manner. The most unique characteristic is that a large part of Thunderer is incorporated into the greater whole as Billy the Kid's techniques, which greatly reduces the mana cost to the point that it is equivalent to an E-rank Noble Phantasm.
As a Servant, Billy can use this technique at a much greater distance than in life. The move is a counterattack, allowing to draw and shoots faster than his opponent can finish an attack they started, but he also can't counterattack guaranteed hits. 
Being a Servant, Billy has superhuman senses that fully allows him to grasp his surroundings and thus his enemy's location regardless of where they are. This allows him to make the most of his three consecutive shots by aiming for their vital points.Fate/Grand Order material III - Profile of Billy the Kid, translated by Kinalvin at Reddit.
==References==
Category:Noble Phantasms
Category:Anti-Unit Noble Phantasms
Category:C++ rank Noble Phantasms
Category:C+++ rank Noble Phantasms
Category:Techniques
Category:Firearms
Category:Noble Phantasms introduced in Fate/Grand Order