Bad upper

Hey Gents, Here's one for a guru to figure out. I had a customer order an AR upper, standard M4 with 14.5 barrel. The upper came in and the customer showed up with his classIII lower, he has a tax stamp and everything is legal. I installed the upper and we stepped out side to test fire. Well, it operated fine on semi but when he went to full auto it would only fire one round. It appeared that the auto sear would not engage the hammer and the hammer would follow the bolt and the round would have a light dimple. As I stated, it worked fine in semi.
We tried 3 other uppers of various manufacturers on the same lower and they all worked fine. We even used the same bolt in all three uppers.
We called the builder of the upper and explained the situation, he said to send it back and he'll take a look at it. Two weeks later I get it back and it did the same thing, we did the same thing with three different uppers and the same lower and had the same results...All three uppers worked and the return upper didn't.
In short, The builder told us to pound sand and the upper was fine and we were idiots...not kidding, his words. I won't mention his name.
I was thinking that maybe the bolt carrier tunnel was a little over sized and the bolt was tilting up on recoil and not pushing the hammer down far enough to engage the auto sear? Reason is because of the unusual wear marks inside top of the receiver. So what do you think:banghead:

I don't think the lower is the problem. We tested three other uppers a 14.5, 10.5 and a 16 inch which all worked fine. We also used only one BCG in each of the uppers to eliminate the BCG problem. Today, I replaced just the upper receiver (had an extra stripped upper on hand), kept the same barrel, gas block, BCG and gas tube. No problems and it cycled fine in semi and auto.
I still feel that the upper was machined incorrectly and has a major carrier tilt problem. Any one know the correct dimensions and the proper way to measure the carrier tunnel in the upper.

Thanks to all those that have replied.

No need to send the lower to the builder because it worked on three diffrent uppers.
It's a DGI.

blackDefense / BAD UPPER

Please educate me on the upper receiver relief cuts for the auto sear and how I can tell the difference.

Thanks for your reply.

BIG THANKS to blackDefense

LOVE IT. Always something new to learn, never pointed this out to us in the armorer course (Colt in Hartford). Yep that was it. Your the Man blackDefense. Now that explains why the sear wasn't catching the hammer, it must of been binding on the top of the auto sear and the upper receiver.

THANK YOU!!!!....

It was from "R" Guns. The owner Roger basically called me an idiot. He stated that he goes off of the 1961 blue prints and it's all to spec. The barrel wasn't milspec because it didn't use tapper pins for the A frame, nor where any cuts made to use tapper pins. He used four set screws and red lock tight between the barrel and the A frame. I couldn't pull off the A frame and had to remove the gas tube and slide it past the A frame in order to use the barrel wrench...pain in the butt.

Thanks for your help.

Yep. I mentioned that to him. I didn't have specifics but did mention that in 1961 they didn't have Forward asst or picatinny rails and M4 cuts. Still called me a few names after that. Oh well. I'll stick to AIM Surplus and BCM. I do a lot of Dura coat and Gunkote on AR platforms and never had an A frame on so tight that I couldn't get it off until this one.