Going straight to the question at hand: If the Medium Weaver Quad Lock rings will cater to a the World Class 50mm bell...while I have no proof with the 3-9x50 model especially, I kind of doubt it. I use the Quad Locks on a couple of my rifle and with my Handis I use the Highs. Biggest bell on mine is 44mm with AO, meaning it's close to 50mm. Main problem is the rear variable or rear bell on the scope, getting it to clear the hammer. If your determined to do it though, get the Mediums form Walmart, and see if they clear. If not go back and exchange them for Highs. While not the greatest rings by anymeans, if carefully adjusted, they work just fine.
-Aaron