Since I gotta be a spoilsport (part of the job) just thought I'd reiterate Gary and Archangel's points -- homebrew auto conversions are a good way to end up in jail if you're lucky... and in the hospital with missing body parts if you're not.
Here follows the "dont try this at home" speech.
I once met a man shooting in the desert who modified his AR in a similar fashion. (No Mr. ATF man, at this point I couldn't even begin to describe him other than short, fiftysish, and in one of the many informal shooting spots in rural Idaho. Good luck finding him.)
Anyhow... at the time I thought his solution to the problem was a little crude, but clever. Knowing what I know now about the AR lockup.... it's a miracle he hadn't blowed himself up yet.
So, for all the antis like to talk about how easy our Evil Black Semi-Autos are to convert to FA.... no, it ain't that easy. Sure you might be able to engineer a runaway gun (like the dude who accidentally stuck his SKS firing pin in backwards and ended up putting a round through his brainpan when he dropped the runaway rifle in surprise).. but unless you really know what you're doing, and have access to a good machine shop and the right parts it's not easy to make a good, reliable, SAFE full-auto.
(And if you have access to aforementioned shop and knowledge, you could make your own FA from scratch... coming soon to a neighborhood near you if we ever go the British way of gun control. )
Finally, being all law-abiding and such, I have to say that THR is not a place to discuss "how to" for breaking the law, stupid as the law is.
Thus ends the "dont try this at home" speech. Have a nice day.
-K