Alberto, its has been the issued backup for some branches in the military. I’m not positive on who or what time periods but I think some people still use them.
5.56 only usually 855 and 193. I had a good setup with it running an H buffer and when my buddy tried to shoot wolf in it it was short stroking. I told him I’d swap the buffer out but he didn’t really care as he only wanted to fire a couple rounds.
It may have worked with a carbine buffer, I don’t know.