Not certain about Ruger shotguns, but everybody else uses a threaded bead.... usually a 6-48 thread on US guns (some may use an 8-40.... not sure) and a metric thread (don't recall which diameter and pitch, offhand) on European and Japanese guns. Some really old guns had swaged beads, but I think that went away about the turn of the last century.
They are installed with Blue Loctite (or the equivalent), and will require a hot air gun (or high-powered hairdryer) to release the loctite..... then, they simply unscrew. A pair of small pliers with thin rubber pads on the jaws usually does the trick.