look, the AR is a "system" and there are many things that could be close to their tolerance limit and be causing a failure together whether any one of them alone would not.
overgassing normally means too much gas. like, the port in the barrel is too large. or too close to the chamber. it's possible but unlikely in this situation.
the overgassing problem people USUALLY experience is when you have a carbine length gas where the port is closer to the chamber. that causes the gas to act on the BCG faster than normal. which in turn tries to open the bolt while the chamber pressure is still high and the brass is still stuck to the walls. in this case, the extractor can slip off. putting an o-ring inside the extractor along with the normal insert and spring can help. but it is really treating a symptom of a poor combination of parts.
another thing people do is use a heavier buffer, which also slows the bcg down a wee bit to give the chamber pressure a little more time to drop before opening.
occasionally, you also see some guy who reloads bolt guns in 223 try to use the same powder, which burns too fast for an AR, and will cause similar symptoms. so try some factory ammo here and make sure you're using a powder with the right burn rate.
again, in this situation, the fact that the gas system is mid-length means it's unlikely to be trying to open the bolt too fast. but that is what it sounds like, so who knows?
another possibility is a rough chamber. run a cotton ball in and out of the chamber and see if it snags on anything.
you should be able to move the extractor with a screwdriver but have difficulty with your fingers. so it sounds like yours has a normal level of tension.
also, i know it sound stupid, but does it extract when you hand cycle it? i mean, (point the gun in safe location and put the safety on) just use the charging handle with a full mag and pull it back and let it go several times. do the rounds come out ok by hand?
