My 500 was locking up with Winchester Xpert 3" 1550 fps loads. It would feed Federal 3" steel fine. Finally, I used 4 ought steel wool on the chamber, cleaned it up, seems okay now. However, I've also switched to 2 3/4" Xpert 1550 fps number 3 shot steel, so there was an ammo change. Not sure what did it, but it's working fine, now. I've heard folks gripe about Expert in 870s, too, on a duck hunting site I go to now and then.
YEARS ago, I was using Remington 3" steel in the 500 I had then. All of a sudden, a new box of ammo would hang up on the ejection port, apparently too long for the action. Yet, Federal or Winchester worked fine. At that time, I switched to using Winchester or Federal and never bought another box of 3" Remington steel. Might be fine now, don't know. I'm shooting the high speed 2 3/4" on ducks now cause it works and it works in my Winchester auto with a 2 3/4" chamber, too. I like that stuff. Glad it ain't giving me problems.
Yes, ammo can cause problems. I do prefer the Mossberg primarily because I'm a southpaw shooter. I don't care much for the 870s finish, the express, that is. It's a rust magnet in the salt marshes and bays I hunt. The camo finish on my 500 is quite resilient in those conditions.