The Ruger AC556 and AC556K were the first select fire weapons, aside from a single M16A1, that my old department had for our SOU team.
The original guns were the 180 (?) series with the smaller (lighter) frames. A couple of them were very prone to jamming. A call to Ruger about this got us a "we know about it and we are working on it".
One rifle blew up during a training exercise and slightly injured the operator. It was found that he had a cook off with the round not fully chambered.
Ruger had a recall on this model but failed to notify us. Eventually they replaced the first ones with the 181 (?) series rifles that had beefier receivers.
The selector setup was bad when going from semi to three shot. One never knew if you were going to get two or three rounds the first time. :banghead:
All in all they were pretty good rifles. Accuracy was sufficient but not what I would call great.
The skinny barrels heated up way too fast. They were of no practical use in FA due to their light weight. Semi and 3-shot burst would have been the best setup for these rifles.
We eventually got rid of them and bought MP5's. We kept the original M16A1.