Probably pressed and glued in. I changed a few on a couple guns and had to drill a bigger hole and tap it. Put a soldering iron next to it to heat the rib and see if it loosens it.
Mike,put it in a drill chuck small enough to tighten down on it and slowly turn it by hand.Things change,the old,like me,
870 did have screw in beads.Who knows today.Front sights anr only for thoes that aim,anyway.LOL
I just bought a new 870 and a Trueglo wing stopper fiber optic sight. This sight clips over the vent rib barrel and also has a strip of two sided really stickey foam/tape to hold it in place. I had one of those 'spark" type sights for my old maverick 88 and the gun broke on me shortly after I installed it. But in the short time that I had it I had a hard time keeping it straight. It screwed into the same hole that the front sight came out of. With the clip on sight I feel a little better as the origion al front bead reamins in place and the new fiber optic sight clips in behind it. However I have used it yet, but a friend highly recommends those new sights. I'll be able to tell better after this season how it performs. Here is the link