Pretty normal. They're drilled all the way through on non ribbed barrels.
If it bothers you, unscrew the bead. Get it hot with a propane torch to break the Loctite, and use a small hex socket to remove it. Vise Grips on two flats of the little nut under the ball will work, but you'll probably bugger it up.
Then dress the bottom of the bead's threaded post with a file a few times to knock a fraction of a millimeter off of it so it's flush with the bore, then with a drop of high temp red Loctite, screw it back in.
Personally, I'd pattern it with the chosen load to see if the shot and wad even notice first.
There's hundreds of thousands of Mossberg 500's out there with this and it's not really a problem.