I'm trying to figure out a few things about a gun turret. Can someone help me?

I'm making a gun turret to practice my raycasting abilities, and I am following a tutorial, but I have a problem. A few, actually. First, and biggest, in the findTarget function the script generates a ray, and multiplies it by 500 with a .Unit . What does that do? Second, I want to use that ray to make sure that its not firing at a wall the humanoid is hiding behind, but instead, the humanoid itself. Third, how would I add inaccuracy to my gun? Fourth, the FireBullet() function works, but how do I make the sound sound like its overlapping, it doesn't sound right, its sounds glitchy. Heres the code:

gun = script.Parent

function FindTarget()
    local dist = 100
    local target = nil
    for i,v in pairs(workspace:GetChildren()) do
        local torso = v:FindFirstChild("Torso")
        local humanoid = v:FindFirstChild("Humanoid")
        if torso and humanoid and humanoid.Health > 0 then
            if (gun.Position - torso.Position).magnitude < dist then
                local ray = Ray.new(gun.Position, (gun.Position- torso.Position).Unit*500)
                local hit,position = workspace:FindPartOnRayWithIgnoreList(ray,{torso.Parent})
                if hit == nil then
                    dist = (gun.Position - torso.Position).magnitude
                    target = torso
                else
                    print(hit.Name)
                end
                dist = (gun.Position - torso.Position)
                target = torso
            end
        end
    end
    return target
end

function FireBullet()
    s = workspace.Sound:Clone()
    s.Parent = script.Parent
    s:Play()
    wait(0.6)
    s:Destroy()
end

while wait(0.3) do
    local torso = FindTarget()
    if torso then
        gun.CFrame = CFrame.new(gun.Position, torso.Position)
        local b = Instance.new("Part",workspace)
        b.Size = Vector3.new(0.2,0.2,1)
        b.BrickColor = BrickColor.new("New Yeller")
        b.Velocity = gun.CFrame.lookVector*500
        b.CFrame = gun.CFrame + Vector3.new(0.2,0.2,0.2)
        b.Touched:connect(function(obj)
        FireBullet()
            local human =  obj.Parent:FindFirstChild("Humanoid")
            if human then
                human:TakeDamage(5)
            end
            if obj ~= gun then
                game:GetService("Debris"):AddItem(b,0)
            end
        end)
    end
end