What I'm about to share with you, I've learned from research, corresponding with industry experts, first hand experience troubleshooting & repairing ARs, and from taking the
Semper Paratus AR armorer's course taught by Will Larson. I hope it helps to further understanding.
So an LE6920 Colt should be fine? My son in law had a brand new one that was overgassed still with that Colt spring. Had minor ejector swipe. Heavier buffer and it was gone.
The 6920 isn't over-gassed. Trouble is, it combines a carbine gas system and an extra inch and a half of barrel with an H buffer. The H buffer is borderline too light. That's why Colt uses an H2 buffer in the M4A1.
There is a difference between an over-gassed AR and a buffer that's too light, but it's hard to tell the difference because both give similar syptoms. An AR that uses the right gas port with an under weight buffer will have a carrier speed that's a little high. It will unlock with a little more violence. Recoil will feel sharper. Switching the H buffer for an H2 will slow things down just enough to put operating speed right where it needs to be. Your son's 6920 wasn't over-gassed, it was under buffered. Installing the heavier buffer was the right move.
You can't tell me that there's only one or two springs that make an AR run right.
There are six springs critical to AR function- action, ejector, extractor, trigger return, hammer & disconnector springs. The three I've seen needing replacing the most are the extractor spring, action spring and hammer springs. The one spring I've seen fail the most is the extractor spring. That's why I give them the most attention.
Sometimes a heavier spring and/or o-ring will fix the problem and sometimes not.
The answer isn't a stronger spring. The answer is the
right spring. In the case of the extractor spring, Colt had to increase the spring rate at least three times. Each time was in response to a change that resulted in increased carrier speed. The first time is when they switched to a more energetic powder in the ammo, the second was when the developed the M4 and the third time is when units used the M4 for high volume fire to break contact.
Crane responded to this issue by recommending the addition of an O ring. Colt responded by redesigning the extractor spring. The military refused to approve it because they didn't want to add another part to the supply chain. Finally, Colt got the military to approve the new spring because the O ring was only a temporary fix. O rings flatten, dry out and break apart. If not replaced in a timely manner, the crumbs will foul the extractor and need to be cleaned out.
The O ring is a temporary fix. Any extractor spring that needs an O ring to work properly needs to be replaced. An O ring used with a Colt or Sprinco M4 spring will place too much pressure on the extractor. Too much force will be needed to snap the extractor claw over the rim and accelerate wear.
...when an AR is overgassed it is the gas port...
This is correct.
Either put on an adjustable gas block (which I don't prefer) or go to a heavier buffer.
Heavier buffers will not fix over-gassing. Putting a heavy buffer in an over-gassed AR does nothing more than mitigate the symptoms. Over-gassing must be fixed by reducing the gas drive (smaller gas port, adjustable gas block, etc.)
...All my rigs throw brass at 4 o'clock in a neat pile with no ejector swipe with a standard spring...
The Colt extractor spring is the standard. Colt uses nothing else in their ARs.
...I guess what I'm saying then is it could be a weak ejection spring, which would certainly cause issues, but that is not always the issue, especially in brand new guns.
The extractor spring is what was bad in my brand new PSA. Some time back, we saw a spate of brand new BCM BCGs causing malfunctions caused by extractor springs. Some time after that, BCM quietly admitted they released a bad batch of extractor springs.
A good extractor & spring will not jump the case rim when carrier speeds increase. They will not lose control of the case during extraction. Violent carrier speeds with an in spec extractor & spring will bend or even rip the rim from the case.