You cannot convert an AR into burst mode without the auto-sear, M16 bolt carrier, and the the selector group. Further, in most cases (but not all, depending on the lower specs) you have to machine the lower receiver deeper in the fire control area to accept those components. HTere are a few holes that need to be drilled as well and unless you have a jig to caaomplish this, it’s not that easy.
Further, the fire control group and the sear are NFA items that cannot be purchased without a valid permit.
Understand. Years ago up to about 1986 one could buy the kit with the parts for about 50 buckds provided one was a decent gunsmith. I had a friend who was the best 45 man I ever saw,. He also made pool cues.