Hard to go wrong with this lapping tool. I have used both the AR-15 and the big brother with good results, required nothing more than my cordless drill, upper receiver vise blocks, and vise. Used the lapping compound from my scope ring lapping kit. They are limited in usable life depending on how rough the receivers you lap are.
http://pacifictoolandgauge.com/ar-tools/1139-ar-15-upper-receiver-lapping-tool.html
Loctite 609 for mating the extension and the receiver.
https://tdsna.henkel.com/NA/UT/HNAUTTDS.nsf/web/3E906D6B842166B0882571870000D855/$File/609-EN.pdf