When I joined the NPS as a LE Ranger in the late 80s, the agency had been in the process of slowly upgrading and replacing older firearms. My first posting, I was issued an old Mod. 10. I replaced it with a 686 I bought with my first paycheck. We didn't have any M16A1s, since the upper echelons decided the more popular, heavily visited NPS units had higher priority than us hicks in the sticks. We eventually received Colt "Sporter" AR15s, and then, new Remington 870s to replace military surplus Winchester Mod. 12 Trench Guns (much to my disappointment.)

Later in my career, I assumed armorer duties in another unit which had half a dozen Vietnam-era M16A1s that showed considerable wear, but were completely serviceable. One of them was stamped GM Hydramatic Division. The others were all Colts. As far as I know, all of those rifles are still in the armory, although M4 carbines are now standard issue. The Hydramatic was issued to an officer who has since been promoted to Chief Ranger. He had the opportunity to exchange it for a more modern version, but chose to keep it.