As Turnkey says, there is no law, just the former AG's opinion. Right now there is a Porac sponsored bill to let retired LEO's keep one and only one registered assault weapon that was previously purchased for duty use.
Which brings up another point. As stated above, your agency must allow it. The letter require for purchase and registration states that the rifle shall be used for duty. It is different than the LEO in good standing letter required to waive the 10 days on a non AW purchase.