I think the 12 gauge uses the 400 grn platinum tip bullet designed for the .500 S&W load.
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It would indeed be a .50 cal bullet
As stated above that is exactly correct Paul.
Winchester Platinum tipped 400gr 12ga sabot...
http://www.winchester.com/products/catalog/shotlist.aspx?use=8&gauge=12
From Winchester press release..
Stemming from the popularity of the .500 S&W, Winchester will now offer a 400-gr. Platinum Tip Hollow