What gauge of shells would I need for a Western Field model XNH-480-G?

I inherited this from my mother-in-law and know next to nothing about shotguns.

What gauge of shells would I need for a Western Field model XNH-480-G?
The gauge or caliber should be stamped on the barrel of the gun.......... Look on the barrel................
Reply:10 gauge = .775 inch, 12 gauge = .729 inch, 16 gauge = .662 inch, 20 gauge = .615 inch, 28 gauge = .550 inch.

measure the bore
Reply:then go to your local hunting and fishing store and ask for them or look up bass pro shop and get there phone number and ask or dunns sporting goods ***** sporting good or cabbella sporting goods good luck