I have a 580 series Mini 14 that I have probably fired 4,000 to 4,500 rounds through. I have had no major malfunctions other than one LC case failure. I reload all of my ammo using LC brass and RORG brass for my hunting loads. I have been working on a new load using a 64 gr. bonded bullet. Several weeks ago I increased my powder charge from 25 gr. to 25.5 gr. Two weeks ago, I fired 200 rounds at 50,100, and 200 yards with no problems. When I cleaned my Mini 14 I removed my scope and went to the local gun club to fine tune it after reinstalling the scope. I set my target stands at 50 and 100 yards. Shooting from a bench rest I fired the first, second and third rounds, when I fired the 4th round, it fired 2 rounds instantly, (as in full auto). I removed the magazine opened the breech and visually inspected it, noting nothing unusual. I inserted another 20 round magazine (each had 5 rounds) and fired the first, second and when I fired the third round it double fired again. I continued shooting without further incident. I shot a total of 150 rounds yesterday. This Mini 14 has been a sweetheart for me. Normally, If I set up two target stands at 100 yards, five targets on each and 5 rounds to each target, I'll have 3 or 4 targets with MOA or sub MOA. It all depends on how shakey my 74 year old hands are. Any comments or suggestions as to what may have caused this will be greatly appreciated .