Did not work on the buttstock qd port of my mossberg 940 pro shotgun - Magpul Industries QDM Sling Swivel

Nicely made, quality feel. Worked on the forend QD port, but would not stay in the buttstock QD port. It appears that the retention balls are too close to the swivel body. I tried both QDM swivels in the buttstock port with the same result - they would jam in place (the release tabs would not work) but could be pulled out with some effort with a twist. I tried a cheapie swivel which had more clearance of the retention balls, and it worked fine.