Can’t use till i find a new mag - International Firearm Corporation Ifc Upper Receiver .410 2.5" 18.5" 9rd Flip-up Sights/rail

So picked this upper up excited to use it I have lots of 410 judge ammo and ready to shoot. But can’t load shot or slug shells in mag. Too narrow and can’t get anyone to get me new mag or explain why normal shot shells and judge shells don’t fit.