Update: Went to the range today. Ran 10 rounds through it and took the bolt out and, unfortunately the filing of the retainer head as described above did not work. I brought my tools out to the range and popped out the old retainer and put in a new retainer. I ran 50 more rounds through it and voila, the retainer stays in position.
In any event, I had fun shooting at steel out at 100 and 200 yards. I did check all my primers and no popped primers.
Thanks for the troubleshooting help.