It's not just local police departments. The Feds are just as crooked. After Vietnam the US government took tens of thousands of surplus M16s, chopped the lowers and sold the rest of the parts to dealers who resold them as parts kits. All you had to do was buy an aftermarket lower for about $100 and you could have a kickass AR15 on the cheap. They were still rare back then and you couldn't just go out and buy one off the shelf like you can now. I built one and also built another lower with an autosear just for fun. It's pretty simple machine work and the parts and patterns were available on shotgun news. I could just swap out the lower in 5 minutes and rock and roll.
Long story short, I had an adopted brother ( cousin whose parents died) that I treated like a true little brother and let him play with all my toys. He turned out to be a sorry ass crackhead and eventually became estranged from me and the rest of the family. In his twisted mind he decided to get even and turn me in to the ATF for having an unregistered machine gun. He painted the picture of me being some kind of desperado or something. anyway I got raided by about 15 ATF and DPS. It was pretty ugly. Fortunately I always hid the rock and roll receiver and when they took it from my gun safe it had the semi auto M15 lower on it.
Well, I thought it was fortunate anyway. They sent the gun to Washington and the suckers in the lab declared it to be a full auto even though the lower didn't have an autosear. No way that bolt cycled.
It was a of a mess for awhile.