Update: 2nd round of diagnosis completed. I note the following:
1) The "elevated barrel" test (where I hand insert a round into the barrel and then turn it vertical) was successful with a live round, but only a live round. It failed with one of my snap caps (had to self extract with needle nose pliers) as well as an empty shell. Neither of those had the same weight as a full live round (obviously). In fact, because of that, I believe these snap caps I have (plastic) are worthless because they don't simulate the full weight of a live 5.56 62 gr round.
2) The current jam cleared once I pulled the charging handle with some extra force, and the plastic snap cap ejected. But I'm thinking there may be some damage to the upper now. See this picture and see the aluminum sheer on the lower inside of the upper tract. This pic, of course, is with the BCG and charging handle removed. Note the gas tube is in place and is covered by the key when in full battery.
3) I did some tests with the BCG. I maunally loaded both my snap cap and the empty shell into the barelhead and attempted to close the BCG manually and get it into battery. It would not lock all the way into battery when I tried this.
The following is the BCG in full "unlocked" position (ie. bolt head fully extended).
When I pushed the BCG in after hand loading any/all items, it would only go halfway into battery. Like so: (Now it's on its side, now).
After giving everything a light coat of oil and replacing the parts, I noticed that when I pushed the BCG in with nothing loaded, it went into full battery, as follows (this pic is with BCG and charging handle properly installed into the upper):
I then re-assembled the upper to the lower and tested the charging handle, etc, etc. with no rounds. It also went into full battery (I could observe the BCG through the ejection port). Thinking that the problems may be with the snap cap and empty shell, I then went ahead and loaded one live round from the mag. I pulled the charging handle, and it responded correctly, going into full battery. I then removed the mag, pulled the charging handle, and the loaded live round did eject properly.
Conclusions:
1) Maybe it did need a small coat of oil.
2) I need to buy new snap caps that actually have the full weight of a live 62gr round.
3) If I still have issues after the next round of test firing, then I'll have to look at the buffer and spring next, or replacing the entire upper/BCG asssembly.
More to follow. Thanks.