Merely out of curiousity, I'd like to know how these function (performance wise)
Have any thoughts?

A link to one:
gunbroker.com/auction/ViewItem.asp?Item=3315622

Thanks
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They are all a waste.  If they performed as spec'd, the BATF would ban them in a heart beat.

If you see the video of the spring type in action, look at the receiver closely, you can see that the video  use full auto rifles, and not the device firing the weapon. The dead giveaway is how tight they hold the rifles.
As for the crank type, if you plan on growing a third hand they would work great.

There's one that uses a thumb lever.  If it spins the cam and continues to keep spinning after the end of the stroke, like a wind up top, then it would be a lot of fun.

Or you could just Bump the Rifle!!!!

Save yer $$$

Junk.

ATF Legal Full Auto Conversion:

Tie a bootstring to the charging handle of your AK.  Loop it back around the pistol grip and through the trigger guard accross the face of the trigger.  Hold your AK by the stock.  Tie the loose end of the bootlace to your finger.  PULL.