Lots of great information here guys! Sorry for not being around to answer/address questions or review potential solutions. Had a week of computer trouble.
I'll try to take one thing at a time. A lot to catch up with so if I don't mention you by name know your advice is greatly appreciated.
First, to clear up a couple things:
1) The picture is not my rifle, nor is the ammo show what I was running. I just found this pic on the internet as it showed exactly what was happening.
2) I do not know if the rifle would lock back on an empty mag consistently. For one, I kept topping my mags off. Second - since the problem was intermittent - even the couple times I did run dry don't mean anything since I was having more 'good' rounds than malfunctions.
3) I've put at least 1000 and probably many more rounds through with this exact same ammo (same lot), same buffer, etc. So I think its safe to say buffer weight is not the problem. I'm reluctant to change the buffer to address a problem that I'm sure is caused by something else, unless the temporary switch would serve to help diagnose the issue.
4) it is definitely not a problem with assembly.
As far as mags go - I can't 100% rule out the possibility - but to my mind they're about the least likely possibility. As stated, this happened with multiple mags, which ran fine until the 60-round mark. I was using one 20-round for me, one 20-round for the wife, and had another 30-round for me shooting unsupported. We had no problems with any of them until the issue manifested for the first time, then we started having problems with all of them. Granted, 60 rounds isn't exactly statistically significant, but combined with the new condition of the mags, their pristine condition, and my thorough inspection & testing of all 3 after the incident, I'm fairly certain they're not the problem. I've used these same mags or those exactly like them up till now - as with the ammo - with zero malfunctions.
I have inspected the gas system as much as possible visually. Sounds like the minimal leakage I'm seeing around the gas tube at the block is normal. In all other respects everything checks out fine. It appears secure, aligns well, does not appear to be obstructed (blew air through it with my mouth using rubber tubing over the gas tube, and also looked down it with a light), the gas key is secure, gas rings are gtg, etc. So I think I can rule out that potential issue as well. I have not cleaned the gas tube (ever), because it was my understanding that it was essentially self-cleaning due to gas pressure, and using things like pipe cleaners introduced more of a potential to clog it than carbon would. Whether this is true or not I don't know, but again physical inspection shows it to be fine, and I wouldn't expect <2000 rounds of clean ammo to be enough to gum it up anyway.
No way I can rule out ammo for certain, but I think the nature of the problem suggests otherwise. It ran fine for 60 rounds of the same ammo I've been running for a while now, then started malfunctioning every 5 or so rounds. The sudden onset would seem to suggest ammo is not the issue, so I'm putting it in with magazines as 'possible but highly unlikely'
So thanks to your help, I think its narrowed down to:
1) Lube
2) The extractor spring
Now that I've examined everything, I must admit the bolt assembly looks a little dry. I'm going to follow yalls suggestion here and run it wetter next time. Is a good coating of CLP the best I can do, or would another lube be preferable? How about mobil1 motor oil?
I do have another bolt available, but I'd rather isolate one thing at a time, so if the lube doesn't work I'll take its extractor spring and swap it out with the one in my current bolt assembly.
And I'll bring different ammo just in case. And number my mags