Here is an interesting link on "knock down" power called the Taylor index: http://www.loadammo.com/Topics/July02.htm If my figures for the 12 gauge magnum 1 oz solid slug (454gr) are accurate, you get a value of about 50 on the Taylor index at 100 yards. I measured the inside bore diameter of my 12 gauge and got about 0.7 inches to use in the calculations. This load should outpenetrate just about any rifle out there including the 300.

You have to realize that the Taylor index should only be used to compare big, slow moving bullets, not fast, small bullets. Foot/lbs should be used for modern rifles such as the 300.

Bottom line, if I was hunting Alaskan critters, I would take the 300 and not worry. If I were fishing or just sightseeing, I would take the 12 with solids and not worry. I am not sure about buckshot. Each pellet would have much less ability to penetrate than the 1 oz slug.