You can extend the capacity of Mossberg shotguns but you need to swap out the barrel along with replacing the mag tube due to the way the barrel is attached. I've done this on one of my 500s. Didn't use a 590 barrel though, used a Mossberg 500 8-shot barrel. Mossberg and/or Hastings made an 8-shot barrel. Don't know if they are still being made, neither lists them on their respective websites. It's a simple swap, takes only minutes to remove barrel, add mag extension and replace with new barrel.