After all is said and done,there is ONE avenue that i wonder if you have traveled?/ Inside your magazine tube ring that is soldered on the barrel there on the barrel side are one,or two holes.These holes are what operates the gases that operates your action. If these holes are plugged up with carbon ect.They will make the function of your gun off for sure!! This is probably your problem if all that you have done before is correct. New "o" rings ect. Also don,t over oil your mag tube. hope i helped. vangunsmith. let us know if it fixed it??!!