It's a convoluted story.

When I had the problems, I, as well as several folks I know who are pretty knowledgable about ARs, checked it over and couldn't find anything.

Right about the same time I was planning to have the ban-era muzzle brake cut off, the bbl threaded, and a Phantom attached. I told the gunsmith about the problem, and he said he'd look it over. I sent him the upper only, no BCG or CH.

He did the work, test-fired it on one of his lowers, and said it ran great. He said he only ran a pipe cleaner down the gas tube. (which I'd also done).

I got it back, and immediately took it upper only) to the range along with another complete AR. I swapped the BCG and CH into the fixed upper and it ran fine. I was still confused that it could go from short stroking to running fine with so little attention. I was just glad it was working.

I took it home, assembled it with all its own parts, and it sat in my safe for a few months until I decided to sell it. The new owner got it, took it to the range, and immediately called me to tell me it wasn't functioning. I'm thinking WTF?!?! At first I thought he was trying to scam some $ out of me, but when he described the symptoms, it was the same as before. Again, WTF?!?!?!

I told him the back-story, and offered to give him a full refund. He said he wanted to look at a few things himself, and during his investigation (I think he used a bore light), he found the primer jammed in the gas key. He removed the gas key and drilled the primer out, reassembled, and it ran 100%

It was then that I realized that the gun had run fine for the gunsmith and later for me because other BCGs had been used. Suffice it to say, I felt like an ID10T, but I learned something.