When I was first involved in gold panning and prospecting I carried a Win 1200 12 gauge shotgun with a # 4 in the chamber(blinding shot), a 00 buck, slug, 00buck, slug.
Now with all the choices out there I would use a .45/70 with 405 grain pills, doing 1166 fps(my favorite load) up to 2000 fps. There is few things in North america that, that recipe won't stop. Couple this with a nickle plated or stainless steel Marlin lever and your good to go!
One little trip my betterhalf and I took down in Bella Coola we seen a Sow grizzly and 3 cubs. The cubs were the same size as she was!! I will never go gold panning again when salmon are running! A SxS in this incounter would have a negative outcome.