Welcome to Gunboards forums. Your shotgun should say what guage it is somewhere on the barrels near the rest of the markings. It would be very helpful if you could post clear pictures of the markings, blotting out the serial number if that concerns you. Don't get ripped off by some unscrupulous dealer, it may be worth much more than a Mossberg 500.
Regards, Alan K.
Available for Cabinet level positions, consultation on matters of foreign policy, weddings and bar-mitzvahs. Will work for gold or guns.