#4 start w/gas block all the way open and load 1 round in the magazine at a time. Shut down gas block a bit at a time till gun doesn't lock back on the empty magazine. Then open back up just a bit. Don't worry about where it throws the brass. Worry about if the gun runs or not.
It is extracting so you probably want to put it back as it was.
When working on these guns it is better to only fix/change one thing at a time.
This seems to be a recoil system or gas system problem. Perhaps both.
This shoots well in all my 308's
https://www.theamericanmarksman.com/bullets-1st-762-175-gr-m118-lr--400-ct.html
My bolt gun shoots Nosler 165gr balastic tips over 39.5gr of Varget real well.
Welcome from KY.
I don't think the receivers are the problem. I have a rifle from PSA and a pistol I built off of a PSA receiver set.
How tight did the barrel fit into the upper receiver?
You are on the right path on the recoil system.
My Wilson 6ARC barrel also had an undersized gas port. The 6.5 Creedmoor barrel from Wilson was the proper size.
Your buffer is a little light. 5 and a half oz range buffers seem to work better in the large frame guns.
The complete upper I bought was in 6.5 Creedmoor. Is shot okay but I replaced the barrel w/a Wilson Combat barrel that shoots much better. The gas block wasn't tightened down properly. When I shot it the first time I had to fix that problem. I have a 6.5 Grendel barrel from them that shoots really well also.
I have a Bear Creek BCG that works with a Wilson combat barrel, Areo Precision barrel,and a Bear Creek barrel. Not a side charger but that shouldn't make much difference.