2x is max for everything I've seen so far. Yes, it's a bolt-action shotgun, but it's been in the family for a number of years so I don't want to get rid of it, and I'd like to make it somewhat more useful, if possible.
I seriously doubt you'll have any luck finding a higher capacity mag for your Mossberg. I just don't remember them making anything but 2 round mags. It is what it is...a solid, inexpensive bolt shotgun. I'd expect it to be a good hunting gun. BTW, an option might be to carry an extra mag with you...at least it would speed reloading somewhat.