Good thread. Resurrecting it again. First post here, so thanks for having me!
I just heard and read about the shoestring method recently and relayed the story to an old shooting buddy. He then told me about the staple and showed me one of his videos. Of course I had to do some Googling for the legalities on the staple to see if it fell into the same 'MG' nonsense as the string. All I have found is this thread. Seems to be a fairly unknown method.
From what I can see, it's reliable. He was using factory magazines, and I have no idea if that was a real factor in it, but any Mini owner knows what non-Ruger mags can be like.
Anyways, I brought the video home so I could play with it. It's a bit fuzzy but I was able to load it up, slow it down, and zoom in well enough to try to count the RoF. The only good, solid firing section that had audio and video in the same scene shows me that the rifle fired at least 8-10 rounds per second, or 480-600 per minute, if my math is correct. I believe the AC556 is capable of over 700 per minute, so I don't know what could actually be done with just the staple method.
One thing is for sure- It looks like fun. If for no other reason, it's certainly a file worthy of the Rainy Day Knowledge Bank.
As for safety... I figure if you're forgetful enough that the 'second shot will happen' fact slips your mind that easily, you probably shouldn't be handling a rifle in the first place. I can't imagine someone just setting it up and leaving it that way. But.... Stranger things have happened.
As of now, the video is only on my hard drive, but if anyone shows real interest, I'll load up a light file and post a link.