If you have an old-style ("pre-ban") M-16/AR-15 lower, conversion to auto/burst is pretty easy. Replace the bolt carrier and the trigger/selector group, then add a drop-in sear. You can find details on the Web.
The gobmint considers the sear alone to be a "machine gun" so you will need a license to legally possess/use that part, but, last time I looked, all the other parts are legal. But it's been a while since I looked, so YMMV.
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