The bead on my 20ga 686 Black Onyx was threaded, and I removed it via grasping it with visegrip pliers after first wrapping the bead with a leather strip.
I was fortunate, in that the bead on my gun came out rather easily - YMMV, though.
I found the TruGlo too large, and instead installed a Hi-Viz Spark II removeable FO bead ($8 @ MidwayUSA), which kit comes with 5 different threaded adaptors for various sizes, one of which fit my 686.