I have been building an ar-15  and started with a Matrix Aerospace lower. I assembled the lower,as I have done many times prior and have never had a problem. I finally received my upper from BCM and went to assemble it together and the upper cannot be attached/pinned, as it looks like the lower is out of spec.. I put the upper on my other lowers receivers I have and had no problem. I put some of my other uppers on the lower and they too would not fit.  I bought the lower as a set with a stripped upper but never paid attention to how it fit, as I sold the upper as I went in another direction with the upper than when I first did. It seems the lower(where the upper first makes contact with the lower towards the tube), is not allowing the two to mate. Has anyone else had this problem and does Matrix make their upper/lower receivers this way purposely? I am going to contact my ffl dealer where I bought it from and see what they can do but they are not opened until later today.

I finally got the situation resolved by my dealer, Saint Lucie Shooting Center. The owner and salesman who sold me the lower said they would swap it out or I could upgrade it and that is what I did. The other salesman there said he would take the parts off of my lower I built and I thought he said he would give them back to me in a bag. Instead he disassembled the lower I brought in and then reassembled the newer one within a few minutes. I was very pleased but we were all puzzled by the initial lower. The people there do their best to make it right and I applaud them for doing so.