Yeah I've done it with a staple bent into the number 7 shape. The staple needs to be the correct thickness, I must have gotten lucky w that on my first try.
It worked reliably, I don't remember a stoppage. It's as safe as you are but only YOU should shoot it because you wont forget that it's firing on release. If you decide you don't want a second shot it's a bitc to drop the mag and clear the chamber while holding the trigger back. You can also hold that trigger back and w the off hand pull back the bolt and lock it with that button on top. Not easy either way, I suppose you could switch on the safety and make sure to keep that thing pointed downrange while releasing trigger but I never thought to try that till just now.
Bottom line is that it's not worth the PITA.
It's legal, the rifle fires with each trigger action. One action to pull, one action to release. Someone once wrote to the ATF asking and ATF sent back a letter saying the same thing I just did. I still have a copy of that letter and I would always take it w me when I went out shooting.
Don't spend any money on this, you can do the same thing yourself and most people (like me) remove it after the novelty wears off. You cant hit anything firing that fast so whats the point unless you like burning up ammo.
I got good at pulling trigger, reengage target, release trigger, repeat. And it was just a touch faster rate of fire than semi-auto. But again, whats the point? It's not worth the downsides.