There are programs out there where the feds loan PDs weapons. There are a few small PDs where I live that are on that program. They get the guns (Glocks and M16s) and once a year they must send the feds an invoice with pictures of each gun and its serial number to prove they still have it. I've never worked for a dept on that program so I dont know all the details but a friend of mines does and he was telling me about it.
One year they issued us Govt surplus M16s.....pretty sure they were Vietnam era, lol. Luckily a year later the department issued us new Bushmaster ARs.