I've written a little about the AC-556, but it's hard to find information on Ruger's early competitor to the M16.
The AC-556 is a selective fire variant of the Ruger Mini 14. Occasionally you'll see one in the US at a gunshow, one that was sold early on to a police force when police forces didn't want to look like the 82nd Airborne Division. The Mini 14 was regarded as looking less military, or perhaps less hostile, so some police forces favored it. As I've noted here once before, the Wind River Reservation game warden carried, at one time, a Mini 30, the 7.62x39 variant of the Mini 14.
The selective fire variant is, of course, different in that its an assault rifle and was originally conceived of as a competitor to the M16.
That it did receive military and paramilitary use if known, but murky. The Marine Corps, which didn't like the M16, considered adopting them early on but Ruger couldn't supply the anticipated needs for the Corps so they went on to partially redesign it, leading to the later variants of the M16. The Corps, of course, no longer uses the M16/M4 at all, although the rifle it does use is closely related to it, omitting its gas system.
Some AC-556s were used by Royal Ulster Constabulary in Northern Ireland, which of course is policing use. French police still use a selective fire variant of the Mini 14, produced in France, some paramilitary units in the Philippines used them. The British Bermuda Regiment seems to have used them, although some claim they actually used the Mini 14.
Now it turns out that the United Arab Emirates army used them.
This is typical for the AC-556. You don't tend to find any large military using them, but they were used. But details are nearly impossible to come by.
