The reaction roll determines the attitude of a NPC or a group of NPC towards one or more PCs. It is automatic, done in secret by the GM if the reaction is not already
The influence check is about using a skill or an attribute to influence the narrative by manipulating others. An influence check is unique: its outcome shall be final for the duration of a scene.
Single shot weapons are inherently tricky to use well in tactical situations. The main reason: the reload time prevent them from being reloaded within one GURPS melee. There are, however, ways to use them to your best