How to get a Z value using ScreenPointToRay?

I am currently trying to get a Z value for an FPS (First Person Shooter) by making a gun create a ray positioned to the center of the players screen.

local players = game.Players.LocalPlayer
local mouse = player:GetMouse()
local cam = workspace.CurrentCamera
local camPos = cam:ScreenPointToRay(mouse.X, mouse.Y,  -- Z Value Here --)

I am very clueless on what to do here so please help.

I never used ScreenPointToRay before but I assume its a 2D space coordinates, if you want to get a 3D space coordinates, try Mouse.Hit.Z Since it's a first person, mouse should be right on the center of the screen.