Kelly at the last show I assembled 7 lowers with J&T parts kits. Absolutely no problems with any dims on the lowers. I was very happy with the results. It appears that the guts are mil-spec. I took a look at some of the stuff while building and did not see any out of spec parts. I'm not an expert, but I watch what I'm building, after all it has my "name" on it if it does not work.