A couple of years ago a former member, Glen, who was an officer and Dept armorer in a Department in Ohio did some research on this subject with ATF and posted it to the (old) AR15.com forum. His research indicated that if the personally owned "semiautomatic assault rifle" and "high capacity ammunition feeding devices" were transferred to the Department at some point prior to retirement, then the Department could "gift" the weapon and mags to the retiring or resigning officer. His research also indicated that the spirit of the law was not intended to take away the personally owned LE only weapon and mags upon the officer leaving the Department, as long as the leaving was lawful and not under felony criminal conditions. However spirit has little to do when you get into a cold courtroom.
Personally, I don't see any reason to take a semiautomatic firearm, LE only or not, from a honorably retiring or resigning officer who is not a felon, criminal or a violent offender. I dont see how they all of a sudden become menaces to society, absence any other circumstances, upon leaving the Department.