The picture doesn't look like its any worse than on a standard issue weapon. A mil issued M9 on a mil issued M16 still wobbles some.
I guess thats my question. Is a little rattle normal? Appears by your assesment that its normal. Its not gonna fall off, but it doesnt feel as rigid as what I was expecting. I would consider my entire rig to be mil spec. So thats as good as it gets?