My New Pump Action Shotgun did not come with sling swivels. I had to take matters into my own hands and create utility style sling rings using ordinary bright steel parts that I hardened and added an oiled, black oxide finish to. I utilized the single factory brass swivel insert near the butt stock (10-32 machine threads) and added another bolt and lock-nut that is not visible in the butt-stock. Not counting the warm selenium dioxide solution (for black finish), this was an $8 project.
Magazine Cap Sling Swivel parts are: 10-32 machine screw, 2 small, thin brass washers, 2 large fender washers, a cap/dome nut, the side plate from a #40 or #50 offset roller link (No.50, I think, twist off with two wrenches) and a 5/16" rubber grommet. The thread on the nut is locked with glue. Use a warm selenium dioxide solution from the sporting goods store to blacken parts and rub with oil (don't oil the nut threads before locking with glue).