This is way to bad ass and from what I understand can be added onto any shotty that accepts extention tubs.
http://www.xrailbyrci.com/products.htm
http://www.youtube.com/watch?v=rzqwEP5QC78
http://www.xrailbyrci.com/products.htm
http://www.youtube.com/watch?v=rzqwEP5QC78