C_1

The thread on the tube and trunnion should be the same; mil spec or commercial. The different diameter they are referring to is the actual tube part that the stock slides onto. A commercial stock, will be lose on a mil spec tube, and a mil spec stock wont fit on a commercial tube. So its either the trunnion's or tube's thread thats out of spec. I think its probably the later. Try a quality AR receiver extension (buffer tube) from VLTOR or LMT. A Stag, Spikes or CMMG will do too. Hope this helps.