I don't know of any manufacturers that produce an m16 with a burst mode. You could get a mosfet that will allow you to toggle the burst option though. Check the airsoftmechanics forum. There should be several people making them and to get the burst option you shouldn't have to spend over $40USD (some with free worldwide shipping). Mosfet do a lot of other great things for your AEG too. Better battery life, batter motor life, better response time in semi auto, and better rate of fire. If you do get a burst mosfet you might be able to choose a number between 2-8 for the # of BBs in the burst. Just so you know this number is permanent. It'll be written in the code of your mosfet.
hope it helps,