How do you make your arm's Y axis pointing your mouse / camera's Y axis?

So.. I've lately struggled on how to create a fps gun.

I've tried several different methods like:

[[ ra = {game.Players.LocalPlayer.Character.Torso["Right Shoulder"], game.Players.LocalPlayer.Character.Torso["Left Shoulder"]} cam = Workspace.CurrentCamera

while eq do wait() ra.C0 = CFrame.new(game.Players.LocalPlayer.Character["Right Arm"].Position, cam.CoordinateFrame.lookVector)

end ]]

I've also tried working with the mouse

[[ m = plr:GetMouse()

while eq do wait() ra.C0 = CFrame.new(plr.Character["Right Arm"].Position,m.Hit.p) end ]]

Everything has failed, i really need some help.

Incase you wonder, I need an fps gun such as the "Contact|Alpha"'s guns or USSF's guns or just basically help.