Marv and I both remember Hydramatic M16's.
In his case, it was the rifle he signed for.
In my case, it was a rifle in the platoon.
Because all of our M16's were M16A2's, the one I remember had to have been subject an upgrade kit.
I only recall it because I remember saying, "Hydramatic, like the transmission?"
Yes, like the transmission.
General Motors, Hydramatic Div. made M16A1's in the early '70's same time as Harrington and Richardson. Mostly to show that we weren't tied to Colt as the sole manufacturer, partially to show Colt they didn't have a monopoly on making them.
I wish I'd known more at the time to make some notes.
An M16A1 converted to an M16A2 wasn't much more than the lower remaining from the original gun.
The "AUTO" marking was supposed to be changed to "BURST" and there were a few ways of doing that.
Selector markings were supposed to be added to the right side.
The buffer, spring and receiver extension were retained, but the entire fire control group was replaced, even the parts which were the same.
The complete upper assembly was replaced.
Probably saved $25 a copy.