There were a lot of Loompanics books about converting semi-autos to fully automatic, don't know how much I've trust them though. A crazy survivalist guy those sons I grew up with once showed us how, with a pack of matches, one could make whatever semi automatic rifle he had (which was obviously totally illegal here, but he was an illegal immigrant who'd moved here during the Vietnam war anyway) fire fully automatic for one squeeze of the trigger. Don't know if it actually worked, as he didn't demonstrate it in action, but I think I've seen books on the topic in Delta Books catalogues.
Also, I think the charm of a printed magazine is that, if they're made illegal, if you're in a survival situation, or if you're just caught without one, you would be able to print up some temporary ones.
Also, that was three hundred bullets fired in a row. They had to change out the barrels to prevent warping, so that says something, I think.