At one time a friend of mine had a 500 12 gauge 3". With 2 3/4" shells it was fine. Fire a 3" and it unlocked and ejected the shell. Too much shell for the gun. I decided then they were unsafe. Maybe he just had a defective gun. Those things do happen. But it worried me.
I have had, at different times, two 500s in 20 gauge. Got one in 1980, hunted with it for a year, and then traded it off for a 12 gauge SxS. Got the other one in a multi-gun deal a couple of years ago, and sold it within a week.
Just don't like 'em.
Got two 870s, a 37 and a 97 Winchester. But no more Mossberg shotguns for me.