Got the bolt head separated from the bolt with a brass punch and a small hammer. Took a bit of force. Found a massive burr, on the rear edge of the retaining pin channel. The burr prevented rotation of the bolt, and caused the very rough cycling. I took the burr out with a small file, polished the inside and outside, now the action is very smooth.
I am very disappointed by this, the shotgun barely has 50 rounds through it, and it has never been abused by me. The action was somewhat stiff at first, and got progressively worse. This burr must have been there to start with, and should have be