01-24-2016, 01:46 AM
I lapped all of my uppers. When you first lap you will see a shiny circle begin to form on the outside of the upper where the barrel mates to the upper. If the circle is even all the way around the front of the receiver your upper is square. If you see only a part of the circle shiny your upper is in need of lapping. My SLR, AA, and Yankee Hill uppers were perfectly squared or needed very little lapping but the others were in need of lapping. The only way to measure the barrel would be to put in a lathe and gauge it that I'm aware of. Lapping is an easy process that I do before any build. Many people have had perfect luck without lapping but my 18" Grendel's accuracy improved immensely after I lapped the upper. I was surprised how much some of my uppers were out of spec after lapping .LRRPF52 had pictures of one of his builds during lapping of one of his builds showing the process on this forum.