With an upper receiver lapping/squaring tool. Just because I don't post alot doesn't mean that I haven't built more than my fair share of "ACCURATE" AR's.
http://www.brownells.com/gunsmith-t...16-upper-receiver-lapping-tool-prod20220.aspx.
http://www.brownells.com/gunsmith-t...16-upper-receiver-lapping-tool-prod20220.aspx.