How would I rotate a model that is in the camera?

RunS.RenderStepped:Connect(function()
	LeftArm:SetPrimaryPartCFrame(Cam.CFrame*CFrame.new(0,.5,0))
	--GunMP5:SetPrimaryPartCFrame(Cam.CFrame)
end)
LeftArm:SetPrimaryPartCFrame(CFrame.Angles(math.rad(90),0,0))

^ why does this not rotate the primarypart in a 90 degree angle?